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5.4 Future Applications 195
Layers
(a) (b)
Laser spot
Substrate Mask
Protective
a
Mask
b
Protective
Recording
Protective R a d x
Fig. 5.35. Medium composition with mask layer/protective layer/recording layer
(a), and schematic illustration of partial readout principle of superresolution optical
disk (b)

(a) 1.0 (b) 1.0
d /R d /R
0.9 x 0 0.9 x 0
0 0
0.8 0.8
0.4 0.4
0.7 0.7
0.8 0.8
0.6 1.2 0.6
MTF 0.5 1.6 MTF 0.5 Ordinary
0.2
0.4 0.4 disk
Ordinary
0.3 disk 0.3
0.2 0.2
0.1 0.1
0 0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Spacial frequency (2NA/l) Spacial frequency (2NA/l)
Fig. 5.36. Calculated MTF of system with ideal mask, relative shift δx as parameter
◦
(a), and with 90 rotation different mask shape (b). Reprinted from [5.38] with
permission by Yi hong Wu

by an ellipse with a variable shape and a relative position with respect to the
center of the laser spot.
Simulation has been performed to evaluate the modulation transfer func-
tion (MTF) of this system. First, the point-spread function h(u, v) of the disk
system is calculated and then multiplied by the mask function f(u, v)onthe
disk plane. Here, the mask function is represented as an ellipse with radii a
and b, and with the relative shift of δx with respect to the center of the read-
out laser spot. For simplicity, we assume that the reflectivity of the mask is
0% and that of the aperture 100%.
Figure 5.36a shows the calculated MTF for R 0 =1.1 µm,a = R 0 ,b = R 0 /2
where NA = 0.45,λ =0.78 µmand R 0 =1.22/2NA [5.38]. It is found that the
MTF depends on the relative position of the mask and the superresolution