Two-beam initialization method
for a rewritable multi-layer optical disk
Tsuchino, Akio.1, Nishihara, Takashi.1, Kusada, Hideo.1, Kojima, Rie.1, Nishiuchi, Kenichi.1,Yamada, Noboru.1, Kimura, Katsumi.2, Nakada, Takumi.2 and Sahota, Eiji.2
1
Deigital & Network Technology Development Center, Panasonic Corporation2
Hitachi Computer Peripherals Co., Ltd.
Abstract
The fine feasibility of rewritable triple-layer optical disk with a capacity of 100 GB was previously
demonstrated last year (2009). Here, it was an issue that the reflectivity of triple-layer optical disk was
extremely lower comparing to the case of dual-layer disk, which made the focusing operation very difficult justmerely by utilizing conventional initialization equipment having only one infrared laser beam. We newly
developed an initialization-equipment (initializer) providing two laser beams: one is blue for focusing by whicha large reflectivity can be obtained from the multi-layer optical disk and the other is infrared for initialization
which has sufficiently high output power for initialization at high speed condition. Our prototype triple-layer
optical disk having GeTe-Sb2Te3 phase-change films for every three layer was successfully initialized after theprocesses of stacking three recording layers and coating a cover layer.
Key words: rewritable optical disk, multi-layer, initialization, two-beam optical system, GeTe-Sb2Te3

1. Introduction
In the manufacturing process of the optical disk, reduction of process number is quite essential forincreasing productivity and lowering cost. In the case of multi-layer optical disk, it becomes further importantto reduce the process number especially before forming a cover layer, which will enable to reduce thedeposition of dusts through long process and accordingly bit error rate. In that meaning, it is highly desirable
that initializing process will be performed after completing cover layer. However, we faced a problem that theabove manufacturing process directly could not be applied for the rewritable triple-layer Bu-ray Disc by using
conventional initialization equipment (initializer) providing just one infrared high power laser. This is becausethat the reflectivity of rewritable triple-layer optical disk, particularly in the layers of L1 and L2, generally
tends to be very small, less than 1%, for the infrared laser, which makes the servo control for focusing very
unstable.
Thus, we newly developed an initializer providing two lasers: a blue laser for focus operation, and a highpower infrared laser for the initialization. In this paper, the initializer having two-beam optical head, theinitializing process for triple-layer optical disk using the initializer, and the test results performed on the sample
disk are first described.

2. Experiments
Figure 1 shows a schematic view of the initialization process utilizing the newly developed initializationmethod. The equipment has an optical head that converges two beams: the preceding infrared laser (810 nm)
with stripe shape (1.5m x 80m) for initialization and the following circular blue laser (405nm) for
focusing, through a common objective lens onto the recording layer. Since the initialized area has rather highreflectivity for blue-laser, very stable servo control becomes possible by arranging the blue laser after justbehind the infrared laser.
The schematic of this two-beam optical head is shown in Figure 2. The infrared laser beam is collimated,through a polarization beam splitter (PBS), a cold mirror, and converged onto the revolving optical disk by anobjective lens, while the blue laser beam is independently collimated, through PBS, reflected by a cold mirror,

and also converged on to the disk by the objective lens common for the infrared laser beam. The each reflected
laser beam is isolated by the PBS and enters into focus detector that generates focusing signals. Focusing
mechanisms are prepared for not only blue laser but also for infrared beam, just in case. For increasing theresolution for “layer isolation,” the numerical aperture (NA) of the objective lens is increased from 0.55 forconventional type to 0.75 at infrared wavelength for new one.
The left hand figure in Figure 3 shows the cross section of one example of our rewritable triple-layer
optical disk applied in this experiment[1,2]. In the sample disk, rather GeTe-rich composition in theGeTe-Sb2Te3 pseudo binary line is utilized for every layer. The right hand figure in Figure 3 reveals
compositional dependences of the crystallization speed (without data) and the variations of optical constants,|Δn |+|Δk|, between in the amorphous and crystalline states on the GeTe-Sb2Te3 line, where Δn and Δkrespectively denote the variations of refractive indices and extinction coefficients[3]. Each thickness of theGeTe-Sb2Te3 film applied to L0, L1and L2 L1 is set to 9.0 nm, 7.5 nm and 5.8 nm, respectively.

2. Results and Discussion
Table I and II show the reflectivity of each information layer measured as the triple-layer structure beforeand after initialization, respectively. Every value in the tables is determined by calculation based on thereflectivity and transmissivity of each layer that were obtained experimentally using spectroscope. As can beseen from Table I, L0 can be easily pulled in focus position by detecting the reflection of the infrared laser.
Thus, the initialization of L0 is first performed. In the case of L1 and L2, some technique is required before
starting initialization. As shown in Table I, L1 and L2 show rather low reflectivities both for infrared and bluelasers. Hence, it is necessary at first to make a small crystalline area (trigger area) using infrared laser before
pulling in focus. After making the trigger area, the reflectivities of L1 and L2 become large enough (1.9% and2.6%, respectively) at blue wavelength for focusing. Figure 4 shows the focus error signal from every layer ofL0, L1 and L2 for infrared and blue lasers. As shown in the figure, large and clear “S” signal can be obtainedfor ever layer for the blue laser. Thus, initialization of L1 and L2 can be successively carried out and finally theinitialization of the triple-layer optical disk is completed.
The recording properties of the sample disk were evaluated based on the BDXL standard[4], where thelaser wavelength is 405 nm, the NA of objective lens is 0.85, the shortest mark length is 0.112 m, and thelinear velocity is 7.36 m/s, assuming 33.4 GB/layer of the recording capacity and 72 Mbps for the recording
data-rate[5]. Figures 5 show the dependences of i-MLSE (Integrated Maximum Likelihood Sequence Estimation)and SER (Symbol Error Rate) on write power for each layer[6]. The bottom i-MLSE of 10.5 % (L0), 11.2 %(L1), and 11.5 % (L2) successfully clear the criterions of 12% and SER clear the criterions of 2x10-4.

4. Conclusions
This study was achieved by co-works of Panasonic Corp. and Hitachi Computer Peripherals Co..
Initialization of triple-layer BD-RE can be carried out after a process of deposition of three recording layers andcoating cover layer. The newly developed Hitachi-Computer made initializer (Figure 6) provides two-beam
optical head (blue laser for focusing, infrared for initialization and an objective lens of NA:0.75). A sample disk(Panasonic) having GeTe-Sb2Te3 phase change material films of rather GeTe-rich composition for every layershows good signal quality that successfully clears BD-XL standard.

References[1] Nishihara, T. et al.: International Symposium on Optical Memory 2009 Technical Digest, 40.
[2] Tomekawa, Y. et al.: Phase Change Optical Information Storage, (2009) 68.
[3] Yamada, N. et al.: Proc. SPIE 4342, Optical Data Storage 2001 Santa Fe, (2002) 55..
[4] Blu-ray Disc Rewritable Format (BDXL), pp.17-19.
[5] A. Nakamura et al.: International Symposium on Optical Memory 2009 Technical Digest, 44.
[6] J. Shiraishi et al.: International Symposium on Optical Memory 2009 Technical Digest, 74.

Blue-laser beam
Infrared-laser

Object lensInfrared -laser spot
（For initialization）

Blue-laser spot
（For focusing）

Initialized

Un-initialized

Fig.1. External and movement of initialization equipment for BDXLLD(405nm)
COL
PBS
PowerMonitor

Focus Detector(405nm)Disk

Focus Detector(810nm)OBL

LD(810nm) COL

PBS

ColdMirror

PowerMonitor

Fig.2. Two-beam optical system

Polycarbonate substrate(t1.1mm)0.32µm

GeTe50L0LargerSeparation layer 25µm

Ge (at%)GeTe-Sb2Te3
7.5nm

L1Higher

Separation layer 18µm

L2
Cover layer 57µm

Laser beam

0Te 0Crystallization
speedSb (at%)|Δn|+|Δk|

40Sb2Te3

= 405nm
NA = 0.85

Fig.3. Cross section of triple-layer disk (left) and GeSbTe phase change film applied in it.

TableⅡ. Reflectivity of the initialized sate

TableⅠ. Reflectivity of the uninitialized sate
layer

810nm

405nm

layer

810nm

405nm

L04.5%

0.2%

L00.1%

1.7%

L1

0.8%

0.3%

L10.7%

1.9%

L20.7%

0.3%

L24.4%

2.6%

cover layer

L2 for blue
L1 for infrared
L0blueinfrared
L1 for infrared
L2 for infrared
Fig.4. Focus error signals for the infrared and blue laser of every initialized layer(trigger area).
Although “S” signal of L1 for infrared is very small, that for blue is large and clear.

20
18

L116L0L2

1.E-02

14L0L1SER

iMLSE(%)

1.E-01L2

1.E-031210

1.E-048

1.E-056510152025
Write power (mW)

3035

510152025
Write power (mW)30

Fig.5. Recording properties of the sample disk; the left and right figures show thedependences of i-MLSE and SER on write power.

35

Fig.6. View of the developed Initializer