In this section, we calculate the crosstalk between adjacent recording track for a conventional spot and a super-resolution readout spot. The time dependent readout photocurrent
fsJc;(/1)
is obtained by taking the integral of the product ofl<fux(
...'()'J1)
(light intensity distribution of a spot) andRrec(X,Y)
(the reflectance di tribution) in general,where the integral is carried out over the area of the spot, y is the pickup efficienc defined by the ratio of the light intensity arriving at the photodiode to that reflected from the media and '7 is the photoelectric conversion efficiency of the photodiode. For simplicity, the photocurrents obtained from the areas of mark and land
Im,uk
andl,.�nd,
respectively) are defined by the following equations,
Rnwrk(X,Y)
indicates the reflectance distribution of a recording layer in which a long recording mark exists, andRtaniX,Y)
indicates the reflectance of the land area. The integrals (7-22) and (7-23) correspond to the upper and lower figures of Fig 7-12, respectively. The (peak-to-peak) signal current intensity is defined byI SJG
=II -I I - mark
lund ' (7-�11)The crosstalk current I
rRos
is obtained by replacingFlux< ;(X, Y,t=O)
withF
lux
c;(X
,Y
�D,t=O)
in eqs. (7-22) and (7-23), whereD
m corresponds to the track pitch, and by taking the absolute value of their difference.I
CROS=II' - mark -!'
lund 'I
(7-�G)!'murk= ff dXdY Rmork(X,Y)Fiuxc,(X,Y
+ D,t= 0),
101
(7-'27)
The crosstalk power ratio OcRos is defined by the square of the ratio of the signal current intensity Is1u to the crosstalk current intensity IrRos, and is expressed in dB by eq (7-28).
I C'ROS ()C'ROS =: 20Joglfi --I SJCi
(7-:!.H)
The expressions for a super-resolution spot can be obtained in the same way by repla ing Fluxc; in eqs. (7 -22), (7 -23 ), (7 -26) and (7 -27) with FluxsR.
y
Figure 7-12. Illustration of the mask and the land used in the calculation of equations (7-22) (upper) and (7-23) (lower), respectively.
Figures 7-13 and 7-14 show track pitch dependence of the era talk for a readout spot and a super-resolution spot. The conditions were the same as tho e in Fig. 7-l 0 and Fig. 7-11, and the reflectances of the mark and land were 0.8 and 1.0, respectively. Figure 7-13 shows the dependence of initial transmittance 10. When the initial optical den ity i higher, the crosstalk becomes lower. Figure 7-14 shows the readout power dependence.
There is an optimum readout power for obtaining lower crosstalk. The super-re olution method can increase the track density in comparison with the conventional method
0
� _J -20
<{
1--C/) (/) 0 rr: 0
-40
P=2.5
mW
0.5 1.0
TRACK PITCH
(�m)
Figure 7-13. The crosstalk between adjacent recording tracks at vanous initial transmittance values. The readout power Prep was fixed as 2.5 mW.
0 T0 =0.1
-
(1)""0
...._
� -20
_J <{
(f) 1--(f) 0 0 cr:
-40
0.5 1.0
TRACK PITCH ( �m)
Figure 7-14. Readout power dependence of the crosstalk. The transmittance of the mask layer was fixed to T0=0.1.
7 .3.3 Linear Recording Density
The super-resolution method increases the linear recording density because the diameter along the track direction of the effective spot is narrower than the conventional readout spot, as shown in Figs. 7-9, 7-10 and 7-11. On the other hand, the readout ignal is distorted because the super-resolution spot has an asymmetric shape. In order to know the distortion effect, we calculated the readout waveform and frequency characteristics The conventional readout signal current
!Hf;(t)
can be obtained by the following integral.The super-resolution readout signal can also be obtained as follows, fslgSR
( 1)
= Y'lJJ dXdY Rrec ( X, Y)
F/ux,\'R(X, Y, I),
where
Rrec(X,
Y) is the reflectance distribution of the recording layer.(7-2�))
Figure 7-15 compares the readout output waveforms obtained from a
104
conventional tnethod and a super-resolution readout method for various recording mark lengths. Note that 7'0=1.0 corresponds to the conventional readout method The relative speed of v= 1.4 m/s was adopted and other conditions were the same as before In general, the output signal level of the super-resolution readout method i maller than that of the conventional one because of light absorption of the mask layer. However, it hould be noted that this is not essential from the viewpoint of modulation tran fer function characteri tics. Therefore, the vertical lines of Fig. 7-15 are normalized prop rly. For the short mark length (900 kHz), we can see that a higher output level is obtained by the super-resolution readout method in comparison with the conventional method Figure
7-15 also shows that the waveform obtained by super-resolution readout method contain· a distortion and a phase-shift. These are caused from the asymmetric hape of the ·uper
resolution spot. Figure 7-16 shows 70 dependence of the frequency characteri tic. The higher optical density (lower transmittance) of the mask layer cau es a higher linear recording density.
1'a
=1.0�--r-�����--��
1
Normalized output level300kHz
500kHz
700kHz
900kHz
1'0 =0.1
Figure 7-15. Readout waveform at various recorded mark lengths. The left figure indicates the waveform of the conventional readout method ( l(1= 1. 0 is optically equivalent to no mask layer) and the right figure the waveform of the super-re olution readout method.
l()f)
0
-co
'"0
._
1-:::>
a...
1-:::> T0 =
0
.6
0
w -10> T0 =
1
.0
1-<!:
_J w 0::
400 600 800
FREQUENCY (kHz)
Figure 7-16. Recording frequency dependence of the output level m conventional and super-resolution readout methods at variou initial transmittance values.
7.4 Photochromic Super Resolution Readout
7.4.1 Nonlinear Transmittance Change in a Photochromic Mask Layer
As shown in Fig. 7-9, when the readout spot scans the precolored (initialized) mask layer of a super-resolution disk, only the restricted mask area corre ponding to the backward portion of the spot is bleached by the photo-reaction, because the irradiated light quantity is integrated in this area. Therefore, a smaller effective uper-re oluti n spot can be formed by overlapping the area of the readout spot and the bleached mask area.
To obtain effective super-resolution, it is necessary that the tran mittance of the mask layer vary nonlinearly with the readout light quantity. We theoretically analyzed the relationship between the photon-mode reaction of photochromic molecule and the transmittance change of the photochromic mask layer, and concluded that the nonlinear transmittance change was obtained by setting the optical density (OD) of the rna k layer to a high level in Sec. 7.3. The nonlinear change of the mask layer is expected when
OD � 0.5 in the initial state.
In order to examine the above prediction, we performed the following experiment Figure 7-17 shows the molecular structure and absorption spectrum of the photochromic material (FC-124) used in our experiment. The open ring state converts to the clo ed ring state by irradiation of ultraviolet (300-350 nm) light and the absorption in the r d wavelength region (600-700 nm) increases. On the other hand, the closed ring state converts to the open ring state by irradiation of red light and the absorption in the red wavelength region decreases. Therefore, a red laser beam can be used for super
resolution readout. This material has a high isomerization ratio, enabling a high optical density to be achieved in the thin film state.
-... en
Q) (.) c co ...0
L...
0 en ...0
<!: \
\\_
A
400 600 800
Wavelength (nm)
Figure 7-17. Molecular structure and absorption spectrum of the photochromic material (FC-124) used in the experiment.
At first, we investigated the transmittance change of photochromic film tatically Figure 7-18 shows the sample structure and the experimental apparatus. Three sample were prepared by vacuum evaporation of Ag as a reflective layer, FC -124 as a photochromic layer and CaF2 as a protective layer on a glass substrate. They included a reference sample which had no photochromic layer, a low ()f) sample and a high ()f) sample. The initial OD values were set to be higher than 0.5 for the high()]) ample and lower than 0. 5 for the low OD sample by calibrating the thickness. The thickness and OJ J of the photochromic layer for each sample are given in Table 7-1.
Table 7-I. OD and thickness of the photochromic layer of the sample use d m t e static mvest1gat10n. . h
OD at A. =633 nm (single pass) thickness ( �m)
reference 0 0
Low OD sample 0.3 0.2
High OD sample 0.6 0.4
Beam Splitter
Pulsed Light
(A.
=633nm)PIN Photodiode
�
Objective Lens
Digital
Oscilloscope
.--------=---.
�
Protective Layer ( CaF2)I�
Photochromic Layer (FC-124)L___ ________
__l.,
Reflective Layer (Ag)Glass Substrate SAMPLE
Figure 7-18. Sample structure and experimental apparatus for static investigation of the transmittance change of photochromic film usmg a
single laser pulse.
Pulse Width 50 J.l. s
< >
\
Nonlinear ChangeReference
Low 00 sample (00=0.3)
High 00 sample (00=0.6)
Figure 7-19. Relative reflectance change obtained by static investigation of photochromic films.
These samples were initialized by irradiation of UV light (Hg-lamp). A sing! light pul e from a HeNe laser
(A
=633 nm) was used to irradiate the samples and the corresponding reflected light was detected by a pin photodiode. Relative reflectance change were measured by observing a waveform displayed on a digital oscilloscope. The pul e width was 50 �s and the peak power was 1 mW. Figure 7-19 shows the waveform di, played on the digital oscilloscope. The reference sample has a rectangular wa eform that corresponds to no reflectance changes. An increase in reflectance was ob erved for the photochromic film samples according to photo-bleaching of the photochromic layer. An apparent nonlinear change is observed for only the high ()f) sample7 .4.2. Application to Read-Only Disks
Super-resolution readout was examined for read-only optical disks Figure 7-20 hows the SEM photographs of disk substrates. The specification of the di k ub trat used in the experiment are summarized in Table 7-II. Disk substrate 1 ha a recording density similar to the conventional compact disk and disk substrate 2 ha a higher recording density (shortest (3T) pit: 0.48 �m, track pitch· 0.85 �m) An eight to foUI1een modulation (EFM) signal was recorded on each disk. Figure 7-21 ho\\. the disk structure. The photochromic mask layer (FC-124) was prepared by a vacuum evaporation method and the reflective layer was overcoated on it by the same method. The ()}) of the mask layer was set at 0. 5 in the colored state at a wavelength
A
of 685 nm, by calibrating the thickness. Figure 7-22 shows the schematic diagram of the apparatu for the super-resolution readout. The readout pickup had a laser diode(A.
=685 nm) and an objective lens (NA=0.55). In order to initialize the mask layer, ultraviolet (V)
light wa, directed to a location on the disk using an optical fiber from a Hg lamp. The other readout conditions were as follows: the readout laser power wa 1 0-2.4 mW, the1V
light power was about 10 mW/cm2 and the relative speed was 0.7-1.4 m/ Since the areal density of the coloring light intensity was low, the mask layer was initialized after everal rotations of the disk. The reflectance of the initialized disk was about 1 0 o/o