Optical recording medium having dual information surfaces

1. Field of the Invention:

0001The 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:

0002In 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.

0003Compact 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.

0004The 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.

0005In 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.

0006However, 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.

0007Another 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.

0008The European patent application EP-A-0 706 178 (MATSUSHITA ELECTRIC IND. Co. LTD.) discloses an optical disk having dual information surfaces which are readable by a single optical head, which partially solves the above mentionned problems. It is to be noted that this European patent application falls under Article 54(3) EPC, i.e. the content of this document is only relevant to the novelty of the below-invention.

<u>SUMMARY OF THE INVENTION</u>

0009The present invention is directed to an optical disk which comprises : 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. The optical disk according to the present invention is characterized in that 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, being in the range of 0,59 mm to 0,68 mm ; and a reflectance of the semitransparent reflection film is in the range of 20% to 35%.

0010In a first embodiment of the present invention, the thickness of the first substrate is in the range of 0,56 mm to 0,6 mm, the thickness of the adhesive layer is in the range of 40 µm to 60 µm, and the total thickness of the first substrate and the adhesive layer, is in the range of 0,60 mm to 0,68 mm.

0011In another embodiment of the present invention, a recording material film for recording and reproducing information is formed on the reflection film.

0012In still another embodiment of the present invention, the recording material film is made of a phase-change type recording material.

0013In still another embodiment of the present invention, a label is formed on a surface of the second substrate.

0014In still another embodiment of the present invention, 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.

0015In still another embodiment of the present invention, 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.

0016Thus, 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.

0017These 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.

<u>BRIEF DESCRIPTION OF THE DRAWINGS</u>

0018Figure <b>1</b> is a sectional view of an optical recording medium of Example 1 according to the present invention.

0019Figures <b>2A</b> and <b>2B</b> 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.

0020Figure <b>3</b> is a sectional view of an optical recording medium of Example 2 according to the present invention.

0021Figures <b>4A</b> and <b>4B</b> 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.

0022Figure <b>5</b> is a sectional view of an optical recording medium of Example 3 according to the present invention.

0023Figure <b>6</b> is an enlarged sectional view showing a second optical disk of the optical recording medium of Example 3.

0024Figures <b>7A</b> to <b>7C</b> are graphs showing the measurement results of jitters obtained from trial-manufactured optical recording media.

<u>DESCRIPTION OF THE PREFERRED EMBODIMENTS</u>

0025The present invention will be described by way of examples with reference to the accompanying drawings.

(Example 1)

0026Figure <b>1</b> is a schematic sectional view showing an optical recording medium <b>101</b> of Example 1 according to the present invention. The optical recording medium <b>101</b> is a one-side read type recording medium composed of a first optical disk <b>102</b> and a second optical disk <b>103</b> adhered to each other. Such an optical recording medium can provide excellent performance as a digital video disk (DVD).

0027The first optical disk <b>102</b> includes a disk-shaped first substrate <b>104</b> having a first information surface <b>105</b> where a spiral information track composed of convex and concave portions (pits) is formed. A semitransparent first reflection film <b>106</b> is formed on the first information surface <b>105</b> of the first substrate <b>104</b> by sputtering and the like. The semitransparent first reflection film <b>106</b> is made of gold (Au), aluminum (Al), and the like, for example. The first reflection film <b>106</b> 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 <b>106</b> is preferably in the range of 5 to 20 nm. In Example 1, the thickness of the reflection film <b>106</b> is 10 nm.

0028The second optical disk <b>103</b> includes a disk-shaped second substrate <b>107</b> having a second information surface <b>108</b> where a spiral information track composed of convex and concave portions (pits) is formed. A second reflection film <b>109</b> is formed on the second information surface <b>108</b> of the second substrate <b>107</b> by sputtering and the like. The second reflection film <b>109</b> is made of aluminum (Al) and the like.

0029Information is recorded on the first and second information surfaces <b>105</b> and <b>108</b> 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 <b>109</b> is smaller than the lengths of the pits formed on the second information surface <b>108</b> so that the pits recorded on the second information surface <b>108</b> can be transferred well to the second reflection film <b>109</b>. Specifically, the thickness of the second reflection film <b>109</b> is preferably in the range of 30 to 150 nm. In Example 1, the thickness of the second reflection film <b>109</b> is 50 nm.

0030As shown in Figure <b>1</b>, an adhesive layer <b>110</b> is formed between the first optical disk <b>102</b> and the second optical disk <b>103</b> for adhering the two optical disks. The adhesive layer <b>110</b> 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 <b>102</b> and <b>103</b>. Then, the two optical disks <b>102</b> and <b>103</b> 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.

0031A label <b>111</b> is attached to the surface of the second optical disk <b>103</b>. A hole <b>112</b> (inner diameter: 15 mm) is formed in the center of the optical recording medium <b>101</b> for mounting the optical recording medium <b>101</b> on a driving motor.

0032Now, referring to Figures <b>2A</b> and <b>2B</b>, the reproduction of information recorded on the first and second information surfaces <b>105</b> and <b>108</b> will be described.

0033Figure <b>2A</b> shows the case where information recorded on the first information surface <b>105</b> is read, while Figure <b>2B</b> shows the case where information recorded on the second information surface <b>108</b> is read. A parallel light beam <b>201</b> is converged by a focusing lens <b>202</b> and illuminates the optical recording medium <b>101</b> from the side opposite to the side of the label <b>111</b>, i.e., from the side of the first substrate <b>104</b>.

0034The focusing lens <b>202</b> 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 <b>202</b>.

0035Referring to Figure <b>2A</b>, when information recorded on the first information surface <b>105</b> is reproduced, the light beam <b>201</b> is controlled to be focused on the first information surface <b>105</b> by a known focusing control technique. A reflected light beam <b>203</b> reflected from the first reflection film <b>106</b> is detected by an optical detector <b>205</b> via a splitter <b>204</b>. Thus, the information is read. Referring to Figure <b>2B</b>, when information recorded on the second information surface <b>108</b> is reproduced, the light beam <b>201</b> is controlled to be focused on the second reflection film <b>109</b> by the focusing control technique. A reflected light beam <b>206</b> reflected from the second reflection film <b>109</b> is detected by the optical detector <b>205</b>, and thus the information is read. For the reproduction of information recorded in the optical recording medium <b>101</b>, the wavelength of the light beam <b>201</b> needs to be 650 nm, and the numeral aperture (NA) of the focusing lens <b>202</b> needs to be about 0.6.

0036When information recorded on the first information surface <b>105</b> is reproduced, as shown in Figure <b>2A</b>, the reflected light beams <b>203</b> and <b>206</b> pass through the focusing lens <b>202</b> and are received by the optical detector <b>205</b>. However, the size of the spot of the light beam <b>201</b> formed on the second reflection film <b>109</b> 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 <b>201</b>. Accordingly, the reflected light beam <b>206</b> 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 <b>206</b> passes through the focusing lens <b>202</b>, and the part of the reflected light beam <b>206</b> which has passed through the focusing lens <b>202</b> is not made parallel. Accordingly, the light amount of the part of the reflected light beam <b>206</b> which has reached the optical detector <b>205</b> is small. Thus, the pit information detected by the optical detector <b>205</b> is mostly composed of components modulated by the pits recorded on the first information surface <b>105</b>.

0037When information recorded on the second information surface <b>108</b> is reproduced, as shown in Figure <b>2B</b>, the reflected light beams <b>203</b> and <b>206</b> pass through the focusing lens <b>202</b> and are received by the optical detector <b>205</b>. As in the above case, however, the size of the spot of the light beam <b>201</b> formed on the first reflection film <b>106</b> is in the order of several tens of micrometers, which is considerably large and illuminates a plurality of pits. Accordingly, the reflected light beam <b>203</b> hardly includes individual pit information components. Further, since part of the reflected light beam <b>203</b> which has passed through the focusing lens <b>202</b> is not made parallel, the light amount of the part of the reflected light beam <b>203</b> which has reached the optical detector <b>205</b> is small. Thus, the pit information detected by the optical detector <b>205</b> is mostly composed of components modulated by the pits recorded on the second information surface <b>108</b>.

0038Next, the relationship between the reflectances of the first and second reflection films <b>106</b> and <b>109</b> will be described. In the case where information recorded on the first information surface <b>105</b> is read, as the reflectance of the first reflection film <b>106</b> is higher, the light amount of the reflected light beam <b>203</b> is larger and the quality of the resultant reproduced signal is better. However, in the case where information recorded on the second information surface <b>108</b> is read, as the reflectance of the first reflection film <b>106</b> is higher, the light amount of the light beam <b>201</b> passing through the first reflection film <b>106</b> is smaller. Since the reflected light beam <b>206</b> reflected from the second reflection film <b>109</b> passes through the first reflection film <b>106</b> again, the light amount of the reflected light beam <b>206</b> is further reduced at the reading of the information recorded on the second information surface <b>108</b>. In other words, in the case where information recorded on the second information surface <b>108</b> is read, the light beam <b>201</b> passes through the first substrate <b>104</b>, the first reflection film <b>106</b>, and the adhesive layer <b>110</b> to reach the second reflection film <b>109</b>. The reflected light beam <b>206</b> reflected from the second reflection film <b>109</b> then passes through the adhesive layer <b>110</b>, the first reflection film <b>106</b>, and the first substrate <b>104</b> again. The light thus passes through the first reflection film <b>106</b> twice. Accordingly, if the reflectance of the first reflection film <b>106</b> is high, the light amount of the reflected light beam <b>206</b> becomes small at the reading of the information recorded on the second information surface <b>108</b>. In order to overcome this problem, in the optical recording medium <b>101</b> of Example 1 according to the present invention, the reflectances of the first and second reflection films <b>106</b> and <b>109</b> are set so that a light amount P₂ of the reflected light beam <b>206</b> at the reading of information recorded on the second information surface <b>108</b> is substantially the same as a light amount P₁ of the reflected light beam <b>203</b> at the reading of information recorded on the first information surface <b>105</b>. In this case, the relationship is expressed by k₂ = k₁ / (1-k₁)² where k₁ is the reflectance of the first reflection film <b>106</b> and k₂ is the reflectance of the second reflection film <b>109</b>.

0039The above expression is obtained in the following manner. The light amount P₁ of the reflected light beam <b>203</b> at the reading of information recorded on the first information surface <b>105</b> is expressed by P₁ = P_o × k₁. The light amount P₂ of the reflected light beam 206 at the reading of information recorded on the second information surface <b>108</b> is expressed by P₂ = P₀ × k₂(1-k₁)². Since P₁ = P₂, the above expression is obtained. The reflectances k₁ and k₂ represent the percentage of the reflected light amount with respect to the incident light amount. Specifically, in the optical recording medium <b>101</b> of Example 1, the reflectance of the first reflection film <b>106</b> is in the range of 20 to 35%, while the reflectance of the second reflection film <b>109</b> is 60% or more. The reflectance of the second reflection film <b>109</b> 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 <b>109</b> is made equal to or less than the pit length of the second information surface <b>108</b>, i.e., 0.5 µm, and thus the reflectance is made 60% or more.

0040Next, the aberration of the light beam <b>201</b> focused by the focusing lens <b>202</b> will be described. In the optical recording medium <b>101</b> of Example 1, the optical path length of the light beam <b>201</b> when information recorded on the first information surface <b>105</b> is read and that when information recorded on the second information surface <b>108</b> is read are different by the total of the thickness of the first reflection film <b>106</b> and a thickness t₀ of the adhesive layer <b>110</b>. Since the thickness of the first reflection film <b>106</b> is 0.5 µm or less in the optical recording medium <b>101</b> 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 <b>201</b> focused by the focusing lens <b>202</b> generates an aberration. The aberration increases in proportion to about the fourth power of the NA of the focusing lens <b>202</b>.

0041The relationship between the focusing lens <b>202</b> and a thickness t₁ of the first substrate <b>104</b> will be described. Herein, the thickness t₁ of the first substrate <b>104</b> is considered to include the thickness of the first reflection film <b>106</b> because the thickness of the first reflection film <b>106</b> is negligible in comparison with the thickness t₁ of the first substrate <b>104</b> and the thickness t₀ of the adhesive layer <b>110</b>.

0042In general, the focusing lens <b>202</b> 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 <b>202</b> is designed based on the thickness of the substrate of 0.6 mm. When the optical recording medium <b>101</b> having the first substrate <b>104</b> with the thickness t₁ of 0.6 mm is reproduced by use of this focusing lens <b>202</b>, no problem arises when information recorded on the first information surface <b>105</b> is read. However, when information recorded on the second information surface <b>108</b> is read, the thickness of the adhesive layer <b>110</b> is added to the thickness of the first substrate <b>104</b>. That is, if the thickness t₀ of the adhesive layer <b>110</b> is 40 µm, 40 µm is added to the thickness t₁ of the first substrate <b>104</b>, 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 <b>108</b> is read. This lowers the quality of the reproduced signal. In order to overcome this problem, when the focusing lens <b>202</b> designed for an optical recording medium having a 0.6 mm thick substrate is used, the thickness t₁ of the first substrate <b>104</b> 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 <b>105</b> is read, while it is 0.62 mm when information recorded on the second information surface <b>108</b> is read. In the latter case, the thickness of the substrate is equal to the distance between the incident surface of the first substrate <b>104</b> and the second information surface <b>108</b>. The differences between these thicknesses and the design value 0.6 mm for the focusing lens <b>202</b> are both 20 µm. Thus, substantially the same quality of reproduced signals can be obtained from the first information surface <b>105</b> and the second information surface <b>108</b>. Naturally, variations are generated in the thickness t₁ of the first substrate <b>104</b> and the thickness t₀ of the adhesive layer <b>110</b> in the fabrication process. With the above setting, however, the allowances of these variations are widened.

0043The relationship between the thickness t₁ of the first substrate <b>104</b> and the thickness t₀ of the adhesive layer <b>110</b> will be described in more detail with reference to Figures <b>7A</b> to <b>7C</b>. Figures <b>7A</b> to <b>7C</b> 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 <b>104</b> 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 Figure <b>7A</b>, four types of the first optical disks <b>102</b> having the first substrate <b>104</b> with the thickness t₁ of 0.56 mm, 0.57 mm, 0.62 mm, and 0.63 mm were trial-manufactured. Each of these first optical disks <b>102</b> was adhered with the second optical disk <b>103</b> via the adhesive layer <b>110</b> 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 <b>71</b> shows the jitters obtained when information recorded on the first information surface <b>105</b> is reproduced, while the reference numeral <b>72</b> shows the jitters obtained when information recorded on the second information surface <b>108</b> is reproduced. In Figure <b>7B</b>, four types of the first optical disks <b>102</b> having the first substrate <b>104</b> with the thickness t₁ of 0.56 mm, 0.57 mm, 0.58 mm, and 0.61 mm were trial-manufactured. Each of these first optical disks <b>102</b> was adhered with the second optical disk <b>103</b> via the adhesive layer <b>110</b> 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 <b>73</b> shows the jitters obtained when information recorded on the first information surface <b>105</b> is reproduced, while the reference numeral <b>74</b> shows the jitters obtained when information recorded on the second information surface <b>108</b> is reproduced. In Figure <b>7C</b>, three types of the first optical disks <b>102</b> having the first substrate <b>104</b> with the thickness t₁ of 0.61 mm, 0.62 mm, and 0.63 mm were trial-manufactured. Each of these first optical disks <b>102</b> was adhered with the second optical disk <b>103</b> via the adhesive layer <b>110</b> 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 <b>75</b> shows the jitters obtained when information recorded on the first information surface <b>105</b> is reproduced, while the reference numeral <b>76</b> shows the jitters obtained when information recorded on the second information surface <b>108</b> is reproduced.

0044In 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.

0045In comparison between Figures <b>7A</b> and <b>7B</b>, the following are observed. When the thickness t₀ of the adhesive layer <b>110</b> 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 t₁ of the first substrate <b>104</b> from 0.56 mm to 0.63 mm. On the contrary, when the thickness t₀ of the adhesive layer <b>110</b> is 40 µm, the jitter of the reproduced signal obtained from the first information surface is higher as the thickness t₁ of the first substrate <b>104</b> is thinner. This indicates that, when the thickness t₀ of the adhesive layer <b>110</b> 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 t₁ of the first substrate <b>104</b>. It is therefore expected that, if the thickness t₀ of the adhesive layer <b>110</b> is smaller than 30µm, 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 t₀ of the adhesive layer <b>110</b> should be 30 µm or more.

0046From Figure <b>7B</b>, it is observed that the jitter of the reproduced signal obtained from the first information surface <b>105</b> starts increasing sharply when the thickness t₁ of the first substrate <b>104</b> is in the range of 0.58 to 0.56 mm. This is because, when the thickness t₀ of the adhesive layer <b>110</b> is 40 µm, the influence of the aberration due to the change in the thickness t₁ of the first substrate <b>104</b> 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 t₁ of the first substrate <b>104</b>. It is therefore expected that, if the thickness t₁ of the first substrate <b>104</b> is smaller than 0.56 mm, the jitter of the reproduced signal will sharply increase. Accordingly, the thickness t₁ of the first substrate <b>104</b> should be 0.56 mm or more.

0047The total of the thickness t₁ of the first substrate <b>104</b> and the thickness t₀ of the adhesive layer <b>110</b>, i.e., (t₀ + t₁) is the thickness of a substrate existing when information recorded on the second information surface <b>108</b> is reproduced. From Figure <b>7C</b>, it is observed that the jitter of the reproduced signal starts increasing sharply when the thickness (t₀ + t₁) is in the range of 0.66 to 0.68 mm. The jitter changes substantially parabolically with the change in the thickness t₁ of the first substrate <b>104</b>. It is therefore expected that, if the thickness (t₀ + t₁₎ 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 t₁ of the first substrate <b>104</b> and the thickness t₀ of the adhesive layer <b>110</b>, i.e., (t₀ + t₁) should be 0.68 mm or less.

0048The 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.

0049When information recorded on the first information surface <b>105</b> is reproduced, as the thickness t₀ of the adhesive layer <b>110</b> is larger, the influence of a leak signal from the second information surface <b>108</b> is smaller. As is observed from the comparison between Figures <b>7A</b> and <b>7B</b>, the thickness t₀ of the adhesive layer <b>110</b> is desirably 40 µm or more when information recorded on the first information surface <b>105</b> is reproduced. Further, when the thickness t₁ of the first substrate <b>104</b> is 0.56 mm or more, the jitter of the reproduced signal can be as small as 8%.

0050As is observed from Figure <b>7A</b>, when the distance between the surface of the first substrate <b>104</b> and the second information surface <b>108</b> is 0.66 mm, the jitter of the reproduced signal is 7.5%. From Figure <b>7B</b>, when the above distance is 0.65 mm, the jitter is 6.6%. From Figure <b>7C</b>, 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 <b>108</b> is reproduced, i.e., the total of the thickness t₁ of the first substrate <b>104</b> and the thickness t₀ of the adhesive layer <b>110</b> is desirably 66 mm or less.

0051When the focusing lens <b>202</b> 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 <b>104</b> and the thickness of the adhesive layer <b>110</b> of 0.66 mm, the thickness of the adhesive layer <b>110</b> should be 60 µm or less.

0052From the above description, it is understood that, by setting the thickness t₀ of the adhesive layer <b>110</b> in the range of 40 to 60 µm and the thickness t₁ of the first substrate <b>104</b> 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 <b>105</b> and <b>108</b> are both low, and thus reproduced signals with excellent quality can be obtained.

0053Now, the direction of the spiral tracks of the first and second optical disks <b>102</b> and <b>103</b> will be described. For example, when the spiral track of the first optical disk <b>102</b> is formed from the inner side to the outer side and the spiral track of the second optical disk <b>103</b> 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 <b>105</b> to the second information surface <b>108</b> or from the second information surface <b>108</b> to the first information surface <b>105</b> by focus jumping.

0054Alternatively, when the spiral track of the first optical disk <b>102</b> is formed from the inner side to the outer side, and the spiral track of the second optical disk <b>103</b> 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 <b>105</b> 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 <b>105</b> to the second information surface <b>108</b>. Then, information recorded on the second information surface <b>108</b> 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 <b>102</b> 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 <b>103</b>, the disk is rotated reversely, and the optical head is moved from the outer side to the inner side of the disk.

0055Thus, 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)

0056In 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.

0057Figure <b>3</b> 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.

0058An optical recording medium <b>301</b> of Example 2 is composed of a first optical disk <b>302</b> and a second optical disk <b>303</b> adhered to each other. The same information is stored in the first and second optical disks <b>302</b> and <b>303</b>. The first optical disk <b>302</b> includes a disk-shaped first substrate <b>304</b> with a thickness of 0.6 mm having a first information surface <b>305</b> where a spiral information track composed of convex and concave portions (pits) is formed. A semitransparent first reflection film <b>306</b> is formed on the first information surface <b>305</b> of the first substrate <b>304</b> by sputtering and the like. The second optical disk <b>303</b> stores completely the same information as that in the first optical disk <b>302</b> in the same way. The second optical disk <b>303</b> includes a disk-shaped second substrate <b>307</b> with a thickness of 0.6 mm having a second information surface <b>308</b> where a spiral information track composed of convex and concave portions (pits) is formed. A second reflection film <b>309</b> of aluminum and the like is formed on the second information surface <b>308</b> of the second substrate <b>307</b> by sputtering and the like. The reference numeral <b>310</b> denotes an adhesive layer made of a UV-curable material for adhering the first and second optical disks <b>302</b> and <b>303</b>. The reference numeral <b>311</b> denotes a label for identifying the optical recording medium. The reference numeral <b>312</b> denotes a hole for mounting the optical recording medium <b>301</b> on an optical recording/reproducing apparatus.

0059The reproduction of information recorded on the first and second information surfaces <b>305</b> and <b>308</b> will be described with reference to Figures <b>4A</b> and <b>4B</b>. Figure <b>4A</b> shows the case where information recorded on the first information surface <b>305</b> is read by use of an apparatus designed for an optical recording medium having a 0.6 mm thick substrate. Figure <b>4A</b> is basically the same as Figure <b>2A</b>. That is, a parallel light beam <b>201</b> is converged by a focusing lens <b>202</b> designed for a 0.6 mm thick substrate and illuminates the optical recording medium <b>301</b> from the side of the first substrate <b>304</b>. The light beam <b>201</b> is partly reflected from the first reflection film <b>306</b> and a reflected light beam <b>203</b> is detected by an optical detector <b>205</b> via a splitter <b>204</b>. Thus, the information is read.

0060Figure <b>4B</b> shows the case where information recorded on the second information surface <b>308</b> is read by use of an apparatus designed for an optical recording medium having a 1.2 mm thick substrate. Referring to Figure <b>4B</b>, when information recorded on the second information surface <b>308</b> is reproduced, a parallel light beam <b>401</b> is converged by a focusing lens <b>402</b> designed for a 1.2 mm thick substrate and illuminates the optical recording medium <b>301</b> from the side of the first substrate <b>304</b>. The light beam <b>401</b> passes through the first substrate <b>304</b>, the first reflection film <b>306</b>, the adhesive layer <b>310</b>, and the second substrate <b>307</b> and reaches the second information surface <b>308</b>. The light beam <b>401</b> is partly reflected from the second reflection film <b>309</b>, and a reflected light beam <b>406</b> passes through the second substrate <b>307</b>, the adhesive layer <b>310</b>, the first reflection film <b>306</b>, the first substrate <b>304</b>, and the focusing lens <b>402</b>. The reflected light beam <b>406</b> is then detected by an optical detector <b>405</b> via a splitter <b>404</b>. Thus, the information is read.

0061As shown in Figure <b>4A</b>, when information recorded on the first information surface <b>305</b> is reproduced, the reflected light beams <b>203</b> and <b>206</b> pass through the focusing lens <b>202</b> and are received by the optical detector <b>205</b>. However, the size of the spot of the light beam <b>201</b> formed on the second reflection film <b>309</b> is as large as 1 mm or more, and thus a plurality of pits are illuminated with the light beam <b>201</b>. Also, the reflected light beam <b>206</b> which has passed through the focusing lens <b>202</b> is not made parallel. Accordingly, the light amount of the reflected light beam <b>206</b> which has reached the optical detector <b>205</b> is extremely small. Thus, pit information components obtained from the second information surface <b>308</b> are hardly detected by the optical detector <b>205</b>.

0062As shown in Figure <b>4B</b>, as in the above case, when information recorded on the second information surface <b>308</b> is reproduced, the reflected light beams <b>403</b> and <b>406</b> pass through the focusing lens <b>402</b> and are received by the optical detector <b>405</b>. However, the size of the spot of the light beam <b>401</b> formed on the first reflection film <b>306</b> is as large as 1 mm or more, and thus a plurality of pits are illuminated with the light beam <b>401</b>. Also, the reflected light beam <b>406</b> which has passed through the focusing lens <b>402</b> is not made parallel. Accordingly, pit information components obtained from the first information surface <b>305</b> are hardly detected by the optical detector <b>405</b>.

0063The relationship between the reflectances of the first and second reflection films <b>306</b> and <b>309</b> in the optical recording medium <b>301</b> of Example 2 is basically the same as that of the optical recording medium <b>101</b> of Example 1. In the optical recording medium <b>301</b>, however, information recorded on the second information surface <b>308</b> is not required to be transferred to the second reflection film <b>309</b>. This enables the second reflection film <b>309</b> to be thickened, and thus a reflectance of 90% or more can be obtained.

0064As described above, when the optical recording medium <b>301</b> shown in Figure <b>3</b> is used as an apparatus designed for a 1.2 mm thick substrate, the light beam <b>401</b> passes through the first and second substrates <b>304</b> and <b>307</b>, the first reflection film <b>306</b>, and the adhesive layer <b>310</b>. When the thicknesses of the first and second substrates <b>304</b> and <b>307</b> are 0.6 mm, the total thickness exceeds 1.2 mm by the thickness of the adhesive layer <b>310</b>, though the thickness of the first reflection film <b>306</b> is negligible. This causes aberration. In order to solve this problem, the thickness of the adhesive layer <b>310</b> is preferably several tens of micrometers or less. Alternatively, the thickness of the second substrate <b>307</b> may be thinned by the thickness of the adhesive layer <b>310</b>.

0065The first and second information surfaces <b>305</b> and <b>308</b> may have different formats from each other. For example, information may be recorded on the second information surface <b>308</b> 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 <b>305</b>, an edited version of the movie shortened by cutting part thereof may be recorded on the second information surface <b>308</b>. In this case, since a light beam with a wavelength of 780 nm is used for reproduction from CDs, the first reflection film <b>306</b> 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.

0066As 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 3j

0067In 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.

0068Figure <b>5</b> is an exaggerated sectional view of an optical recording medium <b>501</b> of Example 3. The optical recording medium <b>501</b> is composed of a first optical disk <b>502</b> for reproduction only and a second optical disk <b>503</b> for recording and reproduction adhered to each other. The first optical disk <b>502</b> includes a disk-shaped substrate <b>504</b> with a thickness of 0.6 mm having a first information surface <b>505</b> where a spiral information track composed of convex and concave portions (pits) is formed. A semitransparent reflection film <b>506</b> is formed on the first information surface <b>505</b> of the substrate <b>504</b> by sputtering and the like. The second optical disk <b>503</b> 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 <b>510</b> denotes an adhesive layer for adhering the first and second optical disks <b>502</b> and <b>503</b>. The reference numeral <b>511</b> denotes a label for identifying the optical recording medium. The reference numeral <b>512</b> denotes a hole for mounting the optical recording medium <b>501</b> on an optical recording/reproducing apparatus.

0069In the optical recording medium <b>501</b> shown in Figure <b>5</b>, as in the optical recording medium <b>101</b> shown in Figure <b>1</b>, the first information surface <b>505</b> for reproduction only and the second information surface for recording and reproduction are adhered via the adhesive layer <b>510</b> so that they are apart from each other by about 40 µm. The optical recording medium <b>501</b> is illuminated with a light beam from the side of the first optical disk <b>502</b>.

0070Referring to Figures <b>5</b> and <b>6</b>, the second optical disk <b>503</b> will be described. Figure <b>6</b> is an enlarged exaggerated sectional view obtained by cutting the second optical disk <b>503</b> in a radial direction. A groove track <b>602</b> having convex and concave portions is formed on one surface of a substrate <b>601</b> of the second optical disk <b>503</b>. Then, a reflection film <b>603</b> made of aluminum and the like, a dielectric film <b>604</b> made of SiO₂ and the like, a recording material film <b>605</b>, and another dielectric film <b>606</b> are formed consecutively in this order by sputtering and the like. The reflection film <b>603</b> is disposed to enhance the sensitivity and protect the recording material film <b>605</b> from thermal shock by facilitating heat radiation. The recording material film <b>605</b> 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 <b>604</b> and <b>606</b> are formed to protect the recording material film <b>605</b> from humidity or thermal shock. These dielectric films can be omitted.

0071The 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 overwritten 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.

0072As described above, the optical recording medium <b>501</b> of Example 3 is constructed to receive a light beam from the side of the first optical disk <b>502</b>. The reason is as follows. Information is recorded in the second optical disk <b>503</b> 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 <b>503</b>, in reverse to the case of Example 3, the reflected light amount required at the reading of information recorded on the first information surface <b>505</b> 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 <b>503</b> twice even if the reflectance of the reflection film <b>506</b> is 100%. The above trouble is avoided in the case of the optical recording medium <b>501</b> according to the present invention, which receives a light beam from the side of the first optical disk <b>502</b>. The absorbance of the second optical disk <b>503</b> can be as large as 60%, while the reflectance thereof can be 40%. Thus, when the reflectance of the reflection film <b>506</b> 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 <b>505</b> 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 <b>503</b> is read. When information recorded in the first optical disk <b>502</b> for reproduction only is reproduced, the incident light beam is greatly modulated by pits formed on the first information surface <b>505</b>. Accordingly, a reproduced signal with sufficiently high quality can be obtained even when the reflectance of the first reflection film <b>506</b> is as low as 20%.

0073As described above, since the optical recording medium <b>501</b> of Example 3 is constructed to receive a light beam from the side of the first optical disk <b>502</b> for reproduction only, reproduction from both the first and second optical disks <b>502</b> and <b>503</b> can be conducted with high reliability.

0074Thus, in Example 3, the first and second optical disks <b>502</b> and <b>503</b> 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.

0075Incidentally, a recording material film similar to the recording material film <b>605</b> used in Example 3 may be formed on the second information surface <b>308</b> of the optical recording medium of Example 2. Such a recording material film should be formed between the second information surface <b>308</b> and the second reflection film <b>309</b> of the second optical disk <b>303</b>.

0076Thus, 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.

0077The 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.

0078Alternatively, 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.

0079Alternatively, 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.