One SIL Head Recording/Reading on a Rewritable Dual-Layer Optical
Disk Having High-index Cover and Separation layers
K. Hisada, K. Narumi, H. Habuta, T. Mihara, T. Shiono, S. Furumiya, M. Birukawa and N. Yamada
Digital & Network Technology Development Center, Panasonic Corporation
3-1-1 Yagumo-nakamachi, Moriguchi, Osaka 570-8501, Japan
Phone: +81-6-6906-4837, Fax: +81-6-6906-2421
E-mail: hisada.kazuya@jp.panasonic.com

Abstract
We first demonstrated the dual-layer rewritable recording with single SIL (Solid Immersion Lens)
head that can access both layers. Newly developed optical disk has cover and separation layers having
the refractive index of 1.8, which thickness are 3.0 µm and 3.5 µm respectively, and two phase-change
recording layers, L0 and L1. The SIL head is designed to focus the laser beam on both of L0 and L1
by beam expander. The recording capacity confirmed on the system corresponds to 150 GB.
Keywords: near-field recording, dual-layer, SIL, rewritable, high refractive index

Introduction
Near-filed recording utilizing a SIL (Solid Immersion Lens) optical system has been developedsince 1990s as a possible candidate for the next generation optical disks [1]. It enables a high density
recording even using the basically similar recording media as Blu-ray disks except for the very thincover and separation layers [2].
In 2008, we first demonstrated the recording onto a dual-layer rewritable disk with SIL opticalsystems. On the experiment, we used two optical heads respectively mounting a different shaped SIL
and individually recorded and read on the two layers. In addition, we used the conventional UV resins
with the refractive index 1.5 for the cover and separation layers, so it limited the effective NA(numerical aperture) and the recording capacity was 135 GB.
After that, we investigated to form very thin cover and separation layer with the higher refractive
index. In parallel, we developed an evolved optical head that can access both layers with single SIL. In
this paper, we present structure of the new optical disk and configuration of the new SIL opticalsystem, and overwrite experimental results using them.

Dual-layer disk and SIL optical system
Figure 1 shows the stack structure of the optical disk. A GeSbTe-based phase-change material
film is formed as each recording layer for L0 and L1, which are developed based on Blu-ray Disk
Rewritable (BD-RE) recording films. The cover layer and the separation layer are formed by spin
coating process, and these layers have a refractive index of 1.8 (@ 405 nm in wavelength). The
refractive index is rather larger comparing to that of conventional UV-resin, 1.5, for example used inBD-RE. Thickness of the cover layer and the separation layer are set to 3.0µm and 3.5µm respectively.
Figure 2 is the schematic diagram of recording and readout optics of our SIL optical system. The
refractive index of the SIL (Lens 1) is 2.068. In the previous report in 2008, we adjusted thickness of
the SIL and distance between the SIL and the objective lens (Lens 2) to minimize spherical aberration
for a dual-layer disc by using two optical heads as mentioned above [3]. In the present optical system,
we have used newly developed beam expander composed of three lenses (Lens 3, 4, and 5) as shownin Figure 3. This beam expander is designed to focus the laser beam on both of L0 and L1 withcompensating the spherical aberration by only moving one lens (Lens 5) and fixing the shape of theSIL and the distance between the SIL and the objective lens. This optical system focuses the laser
beam 0 – 8 µm depth from cover layer surface. For the gap servo, an additional optics with 650 nm redlaser is used simultaneously.

Experiments & Results
The total experimental setup was prepared by changing the optical system and adding the gap
servo circuit based on the commercial Blu-ray drive. For evaluating the disk, (1-7) pp modulation code
was adopted and the channel clock frequency was set to 66 MHz. Gap length was set to 20 nm. Using
this setup, recording/reading for both L0 and L1 of the disk was confirmed by moving Lens 5.
Table 1 Recording conditions

Bit Length
Minimum Mark Length
Linear Velosity
Estimated Recording
Capasity (Dual-layer)

64.5nm/bit
86.1nm
2.84m/s
150GB

SIL

70nm/bit
93.1nm
3.08m/s

Blu-ray Disc
110nm/bit
149nm
4.92m/s

128GB

50GBFigures 4 (a) and (b) show dependence of TE signal amplitude on Lens 5 position. The laser
focuses on both L0 and L1 by only adjusting the position of Lens 5.
Figures 5 (a) and (b) show dependence of 9T CNR (carrier to noise ratio) on write power for L0and L1, respectively, at the bit length of 70.0 nm (i.e. minimum mark length was 93.1 nm and linear
velocity was 3.08 m/s). Over 50 dB of CNR was obtained. It means the disk noise is sufficiently lowfor both L0 and L1.
Figure 6 (a) and (b) show eye patterns for L0 and L1 at the bit length of 64.5 nm, where the write,erase, cooling and readout powers were respectively set to 19.0 mW, 8.0 mW, 0.1 mW and 1.2 mW
for L0 and 16.0 mW, 4.7 mW, 0.1 mW and 1.2 mW for L1. The measured jitter values through thelimit equalizer are 11.0 % for L0 and 11.2 % for L1.

Conclusion
We have developed a dual-layer rewritable optical disk with high-index (n=1.8) thin cover andseparation layers and a new SIL optical head system enabling to access the both layers with single SIL.
The recording capacity confirmed on the system will correspond to 150 GB.
It is possible to reduce the bit length if PRML (Partial Response Maximum Likelihood) and muchhigher index resin are adopted. We will develop a disk with much larger capacity, which will be close
to half terabyte.

References[1] S. M. Mansfield et al.: Opt. Lett. 18, (1993) 305.
[2] T. Ishimoto et al.: Jpn. J. Appl. Phys. 48, (2009) 03A015.
[3] M. Birukawa et al.: Technical digest of ISOM/ODS (2008), TD05-152

Fig.1 Layer structure of the dual-layer disk.

Fig.2 Schematic of the SIL optics and servo system.

ET 1.2
fo 1.0ed
uit 0.8
lpmA0.6
deizl 0.4
am
ro 0.2
N0.0

Fig.3 Configuration of beam expander,
objective lens and SIL.

ET 1.2
fo 1.0ed
uit 0.8
lp
mA0.6
dezli 0.4am
ro 0.2
N3.6

3.7

3.8

3.9

4

Lens5 Position [mm]

4.1

0.0
2.2

2.3

2.4

2.5

2.6

Lens5 Position [mm](a) L0(b) L1
Fig.4 Dependence of TE signal amplitude on Lens 5 position

2.7

60505040

40

C/N[dB]

C/N[dB]

60302030201010005101520
51015
Pw[mW]
Pw[mW](a) L0(b) L1
Fig.5. Dependence of 9T C/N on write power at the bit length of 70.0nm(a) L0(b) L1
Fig.6 Eye pattern of (1-7)pp random signal at the bit length of 64.5nm

20