Abstract:
Regarding a signal recorded with a first frequency in a recording area on a specific first recording layer and a signal recorded with a second frequency different from the first frequency in a recording area on a second recording layer which might cause interlayer crosstalk in relation to the first recording layer, a reproducing signal measurement method for a multilayer optical recording medium having at least three recording layers includes reproducing the signal recorded in the recording area on the first recording layer, separating signals recorded respectively with the first and second frequencies, discriminating signal amplitude of the first frequency and a maximum value in signal amplitude of the second frequency, and calculating a signal amplitude ratio between them to measure interlayer crosstalk contained in the signal reproduced from the recording area on the first recording layer. Thus, the interlayer crosstalk quantity caused by unnecessary light can be measured quantitatively.

Description:
INCORPORATION BY REFERENCE 
     The present application claims priority from Japanese application JP 2007-206045 filed on Aug. 8, 2007, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a reproducing signal measuring method for optical recording medium with due regard to interlayer crosstalk of the optical recording medium having a plurality of recording layers, and a signal reproducing apparatus and an optical recording medium having a plurality of recording layers which realize the reproducing signal measuring method. 
       FIG. 1  shows a sectional structure of a conventionally known multilayer optical disk among optical recording media. A principle for selectively recording and reproducing information of each recording layer is schematically shown in  FIG. 1 . In the conventional art, a recording medium includes a total of six layers having information recorded thereon (hereafter referred to as recording layers). The recording medium includes a first recording layer  101 , a second recording layer  102 , a third recording layer  103 , a fourth recording layer  104 , a fifth recording layer  105  and a sixth recording layer  106  in the cited order beginning with an opposite side from the light incidence on the optical disk, i.e., from the bottom side of  FIG. 1 . For accessing recorded information, for example, on the third recording layer  103  by using the six-layer medium having six recording layers, the position of an object lens in an optical recording and reproducing apparatus is controlled and an optical spot  107  is positioned on the third recording layer  103 . At that time, converging rays  108  in the middle of being narrowed down by the object lens are transmitted by the sixth recording layer  106 , the fifth recording layer  105  and the fourth recording layer  104  which are semi-transparent. On the sixth, fifth and fourth recording layers, the luminous flux diameter of the converging rays  108  is sufficiently larger than the diameter of the optical spot  107  on the third recording layer  103 . Therefore, recording information on the sixth, fifth, fourth recording layers  106 ,  105  and  104  which are semi-transparent cannot be resolved and reproduced. Since the luminous flux diameter is large on the sixth, fifth, fourth recording layers  106 ,  105  and  104  which are semi-transparent, the light intensity per unit area becomes relatively small. Accordingly, there is no fear of destroying information on the sixth, fifth, fourth recording layers  106 ,  105  and  104  at the time of recording. In this way, information recording and reproducing on the third recording layer located remote from the incidence side of the irradiation light are implemented without being influenced by the sixth, fifth, fourth recording layers. Information recording and reproducing on other recording layers are conducted in the same way by controlling the position of the object lens. Conditions for thus conducting recording and reproducing on an optical recording medium having a plurality of recording layers without exerting influence upon other layers are described in detail in JP-A-5-101398. 
     In the above-described multilayer disk reproducing method, it is considered that the effect of light attenuation is given by a recording layer located on the irradiation light incidence side of a target recording layer, however, it is not considered that an influence is caused by a phenomenon of multiple reflection of light in a recording layer located on this side of the target recording layer for the recording and reproducing is not taken into consideration. The state in which multiple-reflected light poses a problem in information recording and reproducing will now be described with reference to  FIG. 4 . It is now supposed that the target recording layer of recording and reproducing is an nth layer. Incident light  108  formed of converging rays is applied so as to form an optical spot  107  on the nth layer. At this time, light reflected by an (n+1)st layer located on this side of the target recording layer becomes unnecessary light  402 . After arriving at back of an (n+2)nd layer and being reflected by the back of the (n+2)nd layer, the unnecessary light  402  might be reflected by the (n+1)st layer again, follow nearly the same path of light reflected by the nth layer, and be detected as if it is light reflected by the nth layer. In this case, large interlayer crosstalk is generated. In the present specifications, the “crosstalk” means “interlayer crosstalk.” It is pointed out in Japanese Journal of Applied Physics, Vol. 43, No. 7B, 2004, pp. 4983-4986 and Ushiyama et. Al., Tech. Digest of ODS2006, WDPDP3 that detection of such unnecessary light poses a great problem. 
     SUMMARY OF THE INVENTION 
     In the design of the multilayer optical recording medium, interlayer crosstalk caused by an unnecessary optical spot poses a problem as described above. The reason will now be described. First, since unnecessary rays converge on the (n+2)nd layer to form an unnecessary optical spot, information on the (n+2)nd layer can be optically resolved, and the influence of unnecessary rays overlaps the band of the typical optical recording and reproducing signal and it cannot be separated. Secondly, since return light which is unnecessary light follows nearly the same path as that of the light reflected by the nth layer and detected as if it is light reflected by the nth layer, the original reflected light of the nth layer and the light perfectly overlap on the detector as well. Thirdly, inability to separate light rays on the detector is also a factor which makes it difficult to quantitatively measure an interlayer crosstalk quantity caused by unnecessary light. Heretofore, therefore, there has been no simple effective method for quantitatively measuring the interlayer crosstalk quantity caused by unnecessary light. 
     In view of the above-described problems, an object of the present invention is to provide a method for simply and quantitatively the influence of interlayer crosstalk caused in case that a multilayer optical recording medium has at least three recording layers, or provide a multilayer optical recording medium which makes it possible to simply and quantitatively the influence of interlayer crosstalk caused when the multilayer optical recording medium has at least three recording layers. 
     To achieve the object, the following schemes are used to implement the measurement method according to the present invention. 
     (1) A reproducing signal measurement method for a multilayer optical recording medium having at least three recording layers, a signal being recorded with a first frequency in a first recording area located on an nth recording layer when counted from an opposite side from a light incidence side used when the multilayer optical recording medium is irradiated with light, a signal being recorded with a second frequency different from the first frequency in a second recording area located on an (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light, the first recording area and the second recording area overlapping in a plane, the reproducing signal measurement method including the steps of: 
     reproducing the signal recorded in the first recording area on the nth recording layer; 
     separating signals recorded respectively with the first frequency and the second frequency from the reproduced signal; 
     discriminating amplitude of the signal of the first frequency and a maximum value in amplitude of the signal of the second frequency; and 
     calculating a signal amplitude ratio between the maximum value in the amplitude of the signal of the second frequency and the amplitude of the signal of the first frequency. 
     This makes it possible to quantitatively evaluate interlayer crosstalk caused by an unnecessary optical spot which is generated in the multilayer optical recording medium having at least three recording layers. 
     (2) The reproducing signal measurement method described in (1), further including, before the step of reproducing the signal, the steps of: 
     recording a signal with the first frequency in a first recording area located on an nth recording layer when counted from an opposite side from a light incidence side used when the multilayer optical recording medium is irradiated with light; and 
     recording a signal with the second frequency different from the first frequency in a second recording area located on an (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light. 
     This makes it possible to apply the quantitative evaluation of the interlayer crosstalk caused by an unnecessary optical spot described in (1) to a recording type multilayer optical recording medium. 
     (3) The reproducing signal measurement method described in (1), wherein 
     a signal is previously recorded with the first frequency on an nth recording layer when counted from an opposite side from a light incidence side used when the multilayer optical recording medium is irradiated with light, and 
     a signal is previously recorded with the second frequency different from the first frequency on an (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light. 
     This makes it possible to apply the quantitative evaluation of the interlayer crosstalk caused by an unnecessary optical spot described in (1) to a ROM type multilayer optical recording medium. 
     (4) The reproducing signal measurement method described in (1), wherein the step of discriminating a maximum value is executed in a continuous area of at least 500 μm in a certain direction on the same recording layer. 
     This makes it possible to make the interlayer crosstalk measurement error small. 
     (5) A reproducing signal measurement method for a multilayer optical recording medium having at least three recording layers, the reproducing signal measurement method including the steps of: 
     reproducing a signal recorded with a first frequency in a first recording area located on an nth recording layer when counted from an opposite side from a light incidence side used when the multilayer optical recording medium is irradiated with light; 
     detecting signal amplitude  1  of a reproducing signal from the first recording area located on the nth recording layer in a situation in which a signal is not recorded in a second recording area located on an (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light, the first recording area and the second recording area overlapping in a plane; 
     detecting signal amplitude  2  of a reproducing signal from the first recording area located on the nth recording layer in a situation in which a signal is recorded in a second recording area located on an (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light, the first recording area and the second recording area overlapping in a plane; 
     separating signals recorded respectively with the first frequency and the second frequency from the reproduced signal; 
     discriminating the signal amplitude  1  and a maximum value of the signal amplitude  2 ; and 
     calculating a signal amplitude ratio between the maximum value of the signal amplitude  2  and the signal amplitude  1 . 
     This makes it possible to quantitatively evaluate interlayer crosstalk caused by an unnecessary optical spot which is generated in the multilayer optical recording medium having at least three recording layers. 
     (6) The reproducing signal measurement method described in (5), further including, before the step of reproducing the signal, the steps of: 
     recording a signal with the first frequency in a first recording area located on an nth recording layer when counted from an opposite side from a light incidence side used when the multilayer optical recording medium is irradiated with light; and 
     recording a signal with the second frequency different from the first frequency in a second recording area located on an (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light. 
     This makes it possible to apply the quantitative evaluation of the interlayer crosstalk caused by an unnecessary optical spot described in (1) to a recording type multilayer optical recording medium. 
     (7) The reproducing signal measurement method described in (5), wherein 
     a signal is previously recorded with the first frequency on an nth recording layer when counted from an opposite side from a light incidence side used when the multilayer optical recording medium is irradiated with light, and 
     a signal is previously recorded with the second frequency different from the first frequency on an (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light. 
     This makes it possible to apply the quantitative evaluation of the interlayer crosstalk caused by an unnecessary optical spot described in (1) to a ROM type multilayer optical recording medium. 
     (8) The reproducing signal measurement method described in (5), wherein the step of discriminating a maximum value is executed in a continuous area of at least 500 μm in a certain direction on the same recording layer. 
     This makes it possible to make the interlayer crosstalk measurement error small. 
     (9) A multilayer optical recording medium having at least three recording layers, wherein 
     a signal is recorded with a first frequency in a recording area on an nth recording area when counted from an opposite side from a light incidence side used when light is applied, recorded with a frequency different from the first frequency in a recording area on an (n+2)nd recording layer, and recorded with a constant frequency in recording areas on the same recording layer, 
     a measurement pattern is recorded to measure a maximum value of signal amplitude of a reproducing signal supplied from the recording area on the (n+2)nd recording layer when the signal is reproduced from the recording area on the nth recording layer, and 
     a measurement pattern is recorded to measure signal amplitude of a reproducing signal supplied from the recording area on the nth recording layer when the signal is reproduced from the recording area on the nth recording layer. 
     This makes it possible to obtain a multilayer optical recording medium having at least three recording layers in which interlayer crosstalk caused by a generated unnecessary optical spot can be evaluated quantitatively. 
     (10) The multilayer optical recording medium described in (9), wherein the recording area is continuously formed over at least 500 μm in a certain direction in a plane of the recording layer. 
     This makes it possible to make the interlayer crosstalk measurement error small. 
     (11) The multilayer optical recording medium described in (9), wherein the signal is recorded every other track on the (n+2)nd recording layer. 
     This makes it possible to measure interlayer crosstalk caused by an unnecessary optical spot simply and quantitatively. 
     (12) The multilayer optical recording medium described in (9), wherein a ratio of signal amplitude of the reproducing signal supplied from the recording area on the (n+2)nd recording layer to the signal amplitude of the reproducing signal supplied from the recording area on the nth recording layer is 0.03 or less. 
     This makes it possible to obtain a multilayer optical recording medium which is small in interlayer crosstalk and favorable in recording and reproducing characteristics. 
     (13) The multilayer optical recording medium described in (9), wherein the signal is previously recorded in the recording areas. 
     This makes it possible to obtain a ROM type multilayer optical recording medium having at least three layers such that interlayer crosstalk caused by an unnecessary optical spot which is generated in the multilayer optical recording medium. 
     (14) The multilayer optical recording medium described in (9), wherein the measurement patterns are newly recorded information. 
     This makes it possible to obtain a recording type multilayer optical recording medium having at least three layers such that interlayer crosstalk caused by an unnecessary optical spot which is generated in the multilayer optical recording medium. 
     (15) A signal reproducing apparatus including: 
     an optical unit which has an optical system to irradiate a multilayer optical recording medium with light; 
     a detection unit which detects reflected light of irradiation light emitted from the optical unit; 
     a signal processing unit which conducts processing on the reflected light detected by the detection unit; and 
     a calculation processing unit which conducts calculation on a signal obtained from processing conducted by the signal processing unit, 
     wherein 
     as regards a signal recorded with a first frequency in a first recording area located on an nth recording layer when counted from an opposite side from a light incidence side used when the multilayer optical recording medium is irradiated with light, and a signal recorded with a second frequency different from the first frequency in a second recording area located on an (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light, the first recording area and the second recording area overlapping in a plane, 
     the signal processing unit executes the steps of: 
     reproducing the signal recorded in the first recording area on the nth recording layer; 
     separating signals recorded respectively with the first frequency and the second frequency from the reproduced signal; and 
     discriminating amplitude of the signal of the first frequency and a maximum value in amplitude of the signal of the second frequency, and 
     the calculation processing unit executes the step of: 
     calculating a signal amplitude ratio between the maximum value in the amplitude of the signal of the second frequency and the amplitude of the signal of the first frequency. 
     This makes it possible to evaluate the interlayer crosstalk. It is possible to select only media which are small in crosstalk and favorable in recording and reproducing characteristics by conducting the crosstalk evaluation. 
     (16) A signal reproducing apparatus including: 
     an optical unit which has an optical system to irradiate a multilayer optical recording medium with light; 
     a detection unit which detects reflected light of irradiation light emitted from the optical unit; 
     a signal processing unit which conducts processing on the reflected light detected by the detection unit; and 
     a calculation processing unit which conducts calculation on a signal obtained from processing conducted by the signal processing unit, 
     wherein 
     the signal processing unit executes the steps of: 
     reproducing a signal recorded with a first frequency in a first recording area located on an nth recording layer when counted from an opposite side from a light incidence side used when the multilayer optical recording medium is irradiated with light; 
     detecting signal amplitude  1  of a reproducing signal from the first recording area located on the nth recording layer in a situation in which a signal is not recorded in a second recording area located on an (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light, the first recording area and the second recording area overlapping in a plane; 
     detecting signal amplitude  2  of a reproducing signal from the first recording area located on the nth recording layer in a situation in which a signal is recorded in a second recording area located on an (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light, the first recording area and the second recording area overlapping in a plane; 
     separating signals recorded respectively with the first frequency and the second frequency from the reproduced signal; and 
     discriminating the signal amplitude  1  and a maximum value of the signal amplitude  2 , and 
     the calculation processing unit executes the step of: 
     calculating a signal amplitude ratio between the maximum value of the signal amplitude  2  and the signal amplitude  1 . 
     This makes it possible to evaluate the interlayer crosstalk. It is possible to select only media which are small in crosstalk and favorable in recording and reproducing characteristics by conducting the crosstalk evaluation. 
     According to the present invention, a method and a medium which make it possible to simply and quantitatively measure interlayer crosstalk caused by an unnecessary optical spot on a multilayer optical recording medium having a large number of recording layers are obtained. 
     According to the present invention, it is possible to select and provide only a medium which is small in crosstalk and which has favorable recording and reproducing characteristics by evaluating the interlayer crosstalk. 
     According to the present invention, an apparatus capable of simply and quantitatively measuring interlayer crosstalk caused by an unnecessary optical spot in a multilayer optical recording medium having a large number of recording layers. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a sectional structure of a conventional multilayer recording medium and a principle for conducting recording and reproducing individually on respective layers; 
         FIG. 2  is a diagram showing a structure of a multilayer recording medium according to the present invention and a state in which signals are recorded on respective layers; 
         FIG. 3  is a diagram showing an example of a multilayer recording and reproducing apparatus; 
         FIG. 4  is a diagram for explaining problems of a conventional multilayer recording medium; 
         FIG. 5A  is a diagram showing frequency characteristics of an embodiment of an interlayer crosstalk measuring signal recorded on an optical information recording medium according to the present invention; 
         FIG. 5B  is a diagram showing a reproducing signal of a pertinent layer in an embodiment of an interlayer crosstalk measuring signal recorded on an optical information recording medium according to the present invention; 
         FIG. 5C  is a diagram showing an interlayer crosstalk signal in an embodiment of an interlayer crosstalk measuring signal recorded on an optical information recording medium according to the present invention; 
         FIG. 6  is a diagram showing relations among tracks on respective layers of a multilayer optical recording medium; 
         FIG. 7  is a diagram showing position relations between an interlayer crosstalk measuring signal recorded on an optical information recording medium according to the present invention and reproducing tracks; 
         FIG. 8  is a diagram showing nonuniformity in film thickness between layers of a multilayer optical recording medium; 
         FIG. 9  is a diagram showing relations between a measurement range and a variation of interlayer crosstalk measured values; 
         FIG. 10  is a diagram showing relations between an interlayer crosstalk ratio and reproducing jitter; 
         FIG. 11  is a diagram showing a flow of measurement of interlayer crosstalk caused by an unnecessary optical spot; 
         FIG. 12  is a diagram showing an example showing an interlayer crosstalk measurement signal recording area in a recording medium according to the present invention; 
         FIG. 13  is a diagram showing a circuit which measures interlayer crosstalk caused by an unnecessary optical spot; 
         FIG. 14  is a diagram showing an example of using a DC-signal for an interlayer crosstalk measurement pattern in a recording medium according to the present invention; and 
         FIG. 15  is a diagram for explaining problems of a multilayer recording medium having at least five layers. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
       FIG. 2  schematically shows a sectional structure of a recording medium as one embodiment of the present invention. Here, a recording medium having six recording layers is mentioned as an example. The recording medium includes a substrate  119 , a first recording layer  101 , a second recording layer  102 , a third recording layer  103 , a fourth recording layer  104 , a fifth recording layer  105  and a sixth recording layer  106  in the cited order beginning with the opposite side from the light incidence side used at the time of light irradiation. Each of layer spaces  110 ,  111 ,  112 ,  113  and  114  between recording layers is approximately 10 μm. A cover layer  109  of approximately 50 μm is formed over them. Converging rays  108  are applied from the incidence plane side, and an optical spot  107  is formed on a recording layer. Although each recording layer is typically formed of a reflection film  115 , a protection film  116 , a recording film  117  and a protection film  118 , the recording layer may have a different configuration. In other words, other films may be included, or the protection films may be removed. Although an example in which different recording layers have the same film configuration is shown here, the film configuration may differ from recording layer to recording layer, or the films may differ in material or film thickness. 
     Supposing now that the third recording layer is the nth layer, the third recording layer has a mark string  201  which provides a signal having a first frequency f 1 , recorded thereon and the fifth recording layer corresponding to the (n+2)nd layer has a mark string  202  which provides a signal having a second frequency f 2 , recorded thereon. In the present embodiment, a pattern used to generate a signal having a specific frequency such as a mark string is referred to as interlayer crosstalk measurement pattern. 
     In the present embodiment, interlayer crosstalk caused by an unnecessary optical spot is quantified by using the signal of the first frequency f 1  and the signal of the second frequency f 2 . In an example which will now be described, interlayer crosstalk caused by an unnecessary optical spot which is generated on the fifth recording layer when conducting reproduction on the third recording layer is measured. However, measurement can be conducted in the same way on other recording layers as well. Frequency characteristics of the reproducing signal of the pertinent recording layer and the interlayer crosstalk are shown in  FIGS. 5A-5C . When conducting reproduction on the third recording layer (the nth layer), the reproducing signal and the interlayer crosstalk signal of the pertinent recording layer differ from each other in frequency as shown in  FIG. 5A  and consequently they can be separated from each other. 
     Signal amplitude of the reproducing signal waveform of the pertinent recording layer ( FIG. 5B ) and a maximum value of interlayer crosstalk amplitude ( FIG. 5C ) of the pertinent recording layer thus obtained are detected, and a ratio between them is calculated. There are two main reasons why the maximum value of the interlayer crosstalk signal is detected. 
     A first reason will now be described. The actual track center position in the multilayer optical recording medium deviates slightly every recording layer as shown in  FIG. 6 . A track on the nth recording layer is at a nearly equal distance from a substrate perimeter  601 . For example, a center  605  of the (n+2)nd recording layer deviates from a center  604  of the nth recording layer to the right above in  FIG. 6 . Therefore, a track  602  on the nth recording layer and a track  603  on the (n+2)nd recording layer do not completely coincide with each other. As shown in  FIG. 7 , therefore, tracks are in position relations that they coincide with each other by inclining them a little. In other words, when conducting reproduction on the nth recording layer, the optical spot travels along a track  703  on the nth recording layer and consequently the optical spot obliquely crosses a track  701  on the (n+2)nd recording layer. In other words, if the spot overlaps a mark string  702  which generates a signal, the crosstalk signal strength becomes strong. If the spot deviates from the mark string  702 , however, the crosstalk signal strength becomes weak. In this way, variations are caused in signal level. 
     In addition, in the actual multilayer optical recording medium, the interlayer thickness is not uniform and film thickness nonuniformity is caused microscopically as shown in  FIG. 8 . A part  803  having an average interlayer thickness, a part  802  having an interlayer thickness greater than the average value, and a part  801  having an interlayer thickness less than the average value are generated. A thickness nonuniformity found between an interlayer thickness  112  between the third recording layer and the fourth recording layer and an interlayer thickness  113  between the fourth recording layer and the fifth recording layer is ±approximately 1 μm even if they are produced with comparatively high uniformity, although it depends upon the manufacturing scheme as well. The microscopic interlayer film thickness assumes a value in the range of the average value approximately 1 μm. If reproducing light has a wavelength of approximately 400 nm and an interlayer material has a refractive index in the range of approximately 1.5 to 2.0, interference becomes large in the thickness range of approximately 0.1 μm according to the light interference condition. If the refractive index is 1.55, interference becomes large in the thickness range of 0.13 μm. In other words, it is indicated that the condition under which the spaces between adjacent recording layers become equal and the condition under which the interlayer crosstalk caused by an unnecessary optical spot is maximized appear in some place in the range of thickness nonuniformity. On the other hand, under other conditions, the interlayer crosstalk caused by the unnecessary optical spot becomes lower than the maximum value. Since the interlayer spaces  112  and  113  are formed of the same material, i.e., materials having the same refractive index, the interlayer thickness is considered. However, the interference condition of light determines the interlayer crosstalk. If the interlayer materials differ in refractive index, therefore, the interlayer crosstalk is maximized when the optical path lengths with due regard to the refractive index are equal. 
     The interlayer crosstalk caused by an unnecessary optical spot in an optical recording medium having six recording layers is measured as described below. When reproducing is conducted at a linear velocity of 5 m/sec by using a recording and reproducing apparatus equipped with an optical system having a laser wavelength of 405 nm and a lens NA of 0.85, the frequency of the interlayer crosstalk measurement signal for each recording layer is 1.5 MHz for the first recording layer, 1.3 MHz for the second recording layer, 1.1 MHz for the third recording layer, 0.9 MHz for the fourth recording layer, 0.7 MHz for the fifth recording layer, and 0.5 MHz for the sixth recording layer. In other words, if interlayer crosstalk caused by the fifth recording layer when reproducing is conducted on the third recording layer is measured, f 1  becomes 1.1 MHz and f 2  becomes 0.7 MHz. If these frequencies are frequencies lower than the cut-off frequency, i.e., frequencies at which a signal can be reproduced by using the optical spot, the interlayer crosstalk can be measured even if the linear velocity is different when the laser wavelength and the lens NA are different from those described above. In the present embodiment, frequencies of interlayer crosstalk signals on all recording layers ranging from the first recording layer to the sixth recording layer are changed. However, a combination having the same frequencies may be allowed so long as the nth layer differs from the (n+2)nd layer in frequency. It is desirable that the frequencies differ by at least a reciprocal of the window width. Even if the frequency is the same as that of the data signal, the measurement of the interlayer crosstalk is possible. However, it is desirable that the frequency differs from that of the data signal. Because the interlayer crosstalk can be measured accurately even if an interlayer crosstalk measuring area is provided near a data area, resulting in an efficient format. 
     Measurement results of interlayer crosstalk are put together in Table 1. “Reproducing layer” means a recording layer which is being subjected to reproduction, and “CT layer” means a recording layer which is causing crosstalk. “C level of reproducing layer” means a signal level C 1  of an interlayer crosstalk measurement signal, and “C level of CT layer” means a maximum value C 2  of a signal level of an interlayer crosstalk measurement signal recorded on the recording layer which is causing the crosstalk. A signal level difference ΔC of interlayer crosstalk can be found from a difference (C 2 −C 1 ) between them. If the signal level difference ΔC is found from the signal amplitude, it is found as a ratio (M 2 /M 1 ) between a maximum amplitude value M 2  of the interlayer crosstalk measurement signal recorded on the recording layer which is causing crosstalk and amplitude M 1  of an interlayer crosstalk measurement signal recorded on the recording layer which is being subjected to reproduction. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 C level of 
                   
                   
               
               
                 Reproducing 
                   
                 reproducing 
                 C level of 
                 Interlayer 
               
               
                 layer 
                 CT layer 
                 layer 
                 CT layer 
                 crosstalk 
               
               
                   
               
             
             
               
                 First recording 
                 Third recording 
                 −20 dBm 
                 −45 dBm 
                 −25 
               
               
                 layer 
                 layer 
               
               
                 First recording 
                 Fifth recording 
                 −20 dBm 
                 −50 dBm 
                 −30 
               
               
                 layer 
                 layer 
               
               
                 Second recording 
                 Fourth 
                 −20 dBm 
                 −44 dBm 
                 −24 
               
               
                 layer 
                 recording layer 
               
               
                 Second recording 
                 Sixth recording 
                 −20 dBm 
                 −49 dBm 
                 −29 
               
               
                 layer 
                 layer 
               
               
                 Third recording 
                 Fifth recording 
                 −20 dBm 
                 −48 dBm 
                 −28 
               
               
                 layer 
                 layer 
               
               
                 Fourth recording 
                 Sixth recording 
                 −19 dBm 
                 −49 dBm 
                 −30 
               
               
                 layer 
                 layer 
               
               
                   
               
             
          
         
       
     
     As heretofore described, interlayer crosstalk caused by an unnecessary optical spot on each recording layer can be measured. If there are unnecessary optical spots of at least two kinds as on the first recording layer and the second recording layer, quantification can be conducted as to which layer exerts a great influence. 
       FIG. 9  shows measurement results concerning relations between the measurement range and the interlayer crosstalk measurement error. If the measurement area is harrow, an area where the interlayer crosstalk is maximized does not come in the measurement range in some cases and the measurement error becomes large. The measurement error becomes small to a practical level when the measurement range is made equal to at least 0.5 mm. This indicates that the measurement range needs to be a range of at least 0.1 ms when reproduction is being conducted at a linear velocity of, for example, 5 m/sec. If the measurement range is thus made equal to at least 0.5 mm (500 μm), the measurement error can be made small. 
     For obtaining a better measurement error of 3% or less, it is desirable to set the measurement range equal to at least 0.7 mm. In the present embodiment, a measurement range which is continuous in the circumference direction is provided. If a measurement range which is continuous in the circumference direction is provided, an effect of shortening the measurement time is brought about. Alternatively, the measurement range may have an angle in the circumference direction. In other words, a continuous area extending over a plurality of tracks may be measured. 
       FIG. 10  shows relations between the interlayer crosstalk ratio and the reproducing jitter which is a standard measurement index of reproducing characteristics. The interlayer crosstalk ratio is defined as a value obtained by dividing the interlayer crosstalk signal maximum amplitude value by the reproducing signal amplitude on the pertinent layer. It is appreciated from this that the reproducing jitter is aggravated and the signal quality is degraded as the interlayer crosstalk becomes large. Since the reproducing limit is a level at which the reproducing jitter is 10%, it is necessary to make the interlayer crosstalk ratio equal to 0.03 or less for obtaining favorable recording and reproducing characteristics. In addition, for obtaining jitter of 7% to achieve favorable recording characteristics, it is more desirable that the interlayer crosstalk ratio is 0.02 or less. 
     Measurement may be conducted by using a carrier level at each frequency instead of the signal amplitude. In this case, the interlayer crosstalk is represented by a carrier difference, and defined as a value obtained by subtracting the carrier level of the reproducing signal on the pertinent layer from the maximum value of the carrier level of the interlayer crosstalk signal. A value corresponding to the interlayer crosstalk ratio 0.03 is −30 dB. 
     If the interlayer crosstalk signal is used to inspect whether the medium is good, it is necessary that favorable crosstalk values are indicated in all recording layers to be used. For example, a medium having crosstalk values shown in Table 1 cannot be judged to be a favorable medium, because each of crosstalk caused on the first recording layer by the fifth recording layer and crosstalk caused on the fourth recording layer by the sixth recording layer is at a level that favorable recording and reproducing characteristics are obtained, but crosstalk is too large in other combinations. If each layer can be accessed, however, it is possible to use only a layer in a favorable state instead of all layers. In this example, it is possible to use only the fourth recording layer. 
     Furthermore, in the present embodiment, it becomes necessary to provide a recording area for interlayer crosstalk measurement before the interlayer crosstalk measurement because of a recording type multimedia medium. In other words, the above-described signal reproducing step is preceded by a step of recording a signal with the first frequency (f 1 ) in a recording area on the nth recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light and a step of recording a signal with the second frequency (f 2 ) which is different from the first frequency in a recording area on the (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light. The recording mark string is formed by causing a change such as a phase change, hole opening or a reaction in the recording film by using heat or light caused by laser irradiation. 
     As a result, it becomes possible to apply the quantitative evaluation of the interlayer crosstalk caused by the unnecessary optical spot to the recording type multilayer optical recording medium. 
     A configuration of the apparatus and a reproducing method not described in the present embodiment are made the same as those in second to fifth embodiments. 
     Second Embodiment 
     In a second embodiment, a method for measuring interlayer crosstalk caused by an unnecessary optical spot serving as an embodiment of the present invention will be described.  FIG. 11  shows a flow of the measurement. A recording type multilayer optical recording medium is set in a measurement apparatus, focusing is first conducted on an nth layer when counted from the opposite side from the light incidence side, and recording is conducted with a first frequency (step  1101 , step is abbreviated to S in  FIG. 11 ). Subsequently, focusing is conducted on an (n+2)nd layer, and recording is conducted by using a second frequency which is different from the first frequency (step  1102 ). In addition, reproducing is conducted on the nth layer (step  1103 ). A signal of the first frequency and a signal of the second frequency are separated from the reproducing signal (step  1104 ). Finally, signal amplitude of the first frequency and a maximum value of signal amplitude of the second frequency are discriminated (step  1105 ). An amplitude ratio between the signals is calculated (step  1106 ). An interlayer crosstalk ratio is thus obtained. The step  1101  and the step  1102  may be interchanged in order. In the case of a ROM type multilayer optical recording medium, the first frequency and the second frequency are formed on the medium beforehand. 
     For the measurement, an interlayer crosstalk measurement signal recording area  1205  is provided in an inner circumference part of a recording medium  1  as shown in  FIG. 12 . It is desirable that recording conditions of a recording signal for interlayer crosstalk measurement is recorded in or near the recording area. As a result, it becomes possible to measure the interlayer crosstalk correctly. The recording conditions are conditions for forming an interlayer crosstalk measurement pattern such as the recording frequency, the number of recording tracks, track layout and recording power. In the present embodiment, these conditions are described in a lead-in area which is located on the inner circumference side than the recording area. After these conditions are read out, recording is conducted. It is necessary that an area  1203  where recording is conducted by using the first frequency and an area  1204  where recording is conducted by using the second frequency overlap on a plane. Although the first frequency recording area  1203  and the second frequency recording area  1204  are formed in the inner circumference part, they may be formed in the outer circumference part, or may be formed in the data area. Microscopically, eccentricity occurs in the first frequency recording area  1203  and the second frequency recording area  1204 , and the first frequency recording area  1203  and the second frequency recording area  1204  are rarely aligned with each other perfectly. An area  1206  where the second frequency recording area does not overlap the first frequency recording area appears in a part. 
     These recording areas need not be formed at equal distances from a substrate perimeter  1201  and a clamp area  1202 . It is necessary that the first frequency recording area  1203  and the second frequency recording area  1204  overlap in some places and the first frequency and the second frequency can be reproduced at the same time. If the first frequency recording area  1203  and the second frequency recording area  1204  are formed near the lead-in part, the measurement can be conducted quickly and it is more desirable. By thus recording signals which are different depending upon the layer, it is possible to conduct separation easily to check which layer exerts an influence to give rise to a signal on the basis of a difference between signal frequencies. For frequency separation, it is more efficient to use a frequency separation unit such as a bandpass filter or a spectrum analyzer, or use a dedicated measurement circuit as shown in  FIG. 13 . 
       FIG. 13  is a dedicated circuit which obtains a measurement result  1306  of interlayer crosstalk ratio from a reproducing signal given by a reproducing signal calculation unit  313 . In an example shown here, this circuit is incorporated into a signal processing unit  302  in an apparatus shown in  FIG. 3 . An obtained reproducing signal and a clock obtained from a clock detection unit  322   b  which detects a clock signal such as a wobble are sent to a frequency separation circuit  1301 . They are separated into an interlayer crosstalk component and a signal component of the pertinent layer by frequency separation. As for the interlayer crosstalk component, level detection is conducted by a signal level detection circuit  1302 , a maximum value of a signal over some definite time is detected by a maximum signal level detection circuit  1303 , and data is sent to a signal level calculation circuit  1305 . As for the signal component of the pertinent layer, level detection is conducted by a signal level detection circuit  1304  and data is sent to the signal level calculation circuit  1305 . Calculation is conducted by using the two signal levels, and the measurement result  1306  of the interlayer crosstalk ratio can be obtained. These circuits which conduct calculation processing to find the interlayer crosstalk ratio may not be the circuits in the signal processing unit  302 , but may be incorporated into the reproducing signal calculation unit  313  which is expanded. 
     In the first embodiment of the present invention and the present embodiment, a radial area where a signal of a single frequency is recorded is provided. However, a recording signal on the nth layer may be a DC signal. Not only the mark string but also a pattern used to generate a signal having a specific frequency is called interlayer crosstalk measurement pattern in the present embodiment.  FIG. 14  shows an example in which a DC signal has been recorded. When reproducing is thus conducted on the nth layer, the optical spot obliquely crosses a recorded track and an unrecorded track alternately, as for tracks on the (n+2)nd layer, and consequently frequency characteristics are obtained from the DC signal as well. The interlayer crosstalk ratio may be found by using this. 
     It is thus appreciated that the present invention makes it possible to simply and quantitatively measure the interlayer crosstalk caused by an unnecessary optical spot in a multilayer optical recording medium having a large number of recording layers. 
     A medium configuration, an apparatus configuration and a reproducing method which are not described in the present embodiment are made the same as those in the first and third to fifth embodiments. 
     Third Embodiment 
     Interlayer crosstalk caused by an unnecessary optical spot in a recording medium having at least five recording layers will now be described in the same way as the first embodiment of the present invention with reference to  FIG. 15 . Supposing that the target layer of recording and reproducing is the nth layer, incidence light  108  formed of converging rays is applied so as to form an optical spot  107  on the nth layer as shown in  FIG. 15 . At this time, light reflected by the (n+2)nd layer which is located on this side of the target layer by two layers becomes unnecessary light  402  and arrives at the back of the (n+4)th layer. The unnecessary light  402  reflected by the back of the (n+4)th layer is reflected by the (n+2)nd layer again, and follows nearly the same path as that of light reflected by the nth layer and returns toward the optical pickup, resulting in large interlayer crosstalk. In this way, unnecessary light is formed on a layer not only when there are two layers but also when there are an even number of layers on the light incidence side. 
     In a medium having at least five layers, therefore, interlayer crosstalk is caused on the nth layer by unnecessary light of the (n+2)nd layer and unnecessary light of the (n+4)th layer. In a medium having at least five layers, therefore, it is necessary to change the frequency of the interlayer crosstalk measurement signal recorded on a layer according to whether the layer is the (n+4)th layer, the (n+2)nd layer or the nth layer. In addition, the same holds true even if the number of layers is increased. 
     A medium configuration, an apparatus configuration and a reproducing method which are not described in the present embodiment are made the same as those in the first, second, fourth and fifth embodiments. 
     Fourth Embodiment 
     An example in which reproducing and measurements are conducted on various optical disks by the recording and reproducing apparatus shown in  FIG. 3  is now described. Light rays emitted from a laser light source  334  (having a wavelength of approximately 405 nm in the present embodiment) which is a part of a head  303  are collimated to nearly parallel light rays by a collimator lens  331 . Information such as the laser intensity at the time of reproducing is recorded in a memory  329  in the recording and reproducing apparatus as well. Laser light is controlled by a laser driver  328  on the basis of information such as the laser intensity and timing obtained from a control circuit (microprocessor)  327 . The optical beam controlled by the laser driver  328  is transmitted through a beam splitter  336 , and applied onto an optical disk  301  as converging rays  108  through an aberration correction element  337  and an object lens  330  to form a spot  107 . Reflected light from the disk is led to a servo detector  351  through a beam splitter  336 , a hologram element  339  and a detection lens  333  and led to a signal detector  352  through the beam splitter  336  and a detection lens  332 . Signals from the servo detector  351  and the signal detector  352  are subjected to addition and subtraction processing. Resulting servo signals such as a tracking error signal and a focus error signal are input to a servo circuit  379 . On the basis of the obtained tracking error signal and focus error signal, the servo circuit  379  controls an object lens actuator  378  and a position of the whole optical head  303  to position the optical spot  107  on a target recording and reproducing area. An addition signal of the detector  352  is input to the signal processing unit  302 . The input signal is subjected to filter processing and frequency equalization processing and then digitized in a signal processing circuit  325 . Address information formed on the disk as groove wobbles is detected as differential signals from the divisional detector  352 , and it is input to a wobble detection circuit  322  in the signal processing unit  302 . The wobble detection circuit  322  functions to generate a clock synchronized to a wobble signal and discriminate a wobble waveform. A wobble signal detected by the wobble detection circuit  322  is converted to digital information by an address detection circuit  323 . Thereafter, processing such as error correction is conducted by a decoding circuit  326 , and the wobble signal is detected as address information. On the basis of the detected address information, a start timing signal of recording and reproducing processing is generated to control a user data demodulation circuit  324 . At the same time, address information is sent to the control circuit (microprocessor)  327  as well, and used for, for example, access. Other mechanisms shown in  FIG. 3  are the calculation processing unit  313 , the hologram element  339 , rotation control and an automatic position control unit  376 , a motor  377 , and a high rank apparatus (host)  399 . 
     Heretofore, the recording type medium in which the measurement signal is recorded has been mainly described in the embodiment. The same is true of the ROM type in which recording marks are formed by pits, a recording type in which a measurement signal is added, a rewriting type medium, and their measurement methods as well. For example, if the multilayer optical recording medium is ROM type, a signal is previously recorded in a part where it is indicated that a signal is recorded and reproduced and consequently the step of recording a signal is not necessary. 
     In the present embodiment, jitter measurement has been described. However, a method of recording or reproducing information on a medium of the present invention by using the recording or/and reproducing apparatus shown in  FIG. 3  is also an embodiment of the present invention. 
     A medium configuration, an apparatus configuration and a reproducing method which are not described in the present embodiment are made the same as those in the first to third and fifth embodiments. 
     Fifth Embodiment 
     In a fifth embodiment, the case where the multilayer optical recording medium is ROM type will be described. 
     In the present embodiment, it becomes necessary to provide a recording area for interlayer crosstalk measurement previously at the time of medium fabrication because of the ROM type interlayer medium. In other words, the above-described signal reproducing step is preceded by a step of recording a signal with the first frequency (f 1 ) in a recording area on the nth recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light and a step of recording a signal with the second frequency (f 2 ) which is different from the first frequency in a recording area on the (n+2)nd recording layer when counted from the opposite side from the light incidence side used when the multilayer optical recording medium is irradiated with light. It is inexpensive and desirable to form the recording signal of uneven pits by using a method suitable for mass production such as injection. Alternatively, the recording signal may be formed by causing an irreversible change which cannot be erased, such as a phase change, a reaction or hole opening in the recording film material. 
     As a result, it becomes possible to apply the quantitative evaluation of the interlayer crosstalk caused by the unnecessary optical spot to the ROM type multilayer optical recording medium. 
     A layer configuration of the medium, an apparatus configuration and a reproducing method which are not described in the present embodiment are made the same as those in the first to fourth embodiments. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.