To evaluate the writing performance of the disk with optical switching structure, sample disks “A” and “B” with structures shown in Fig. 4.1.1 (a) and (b), respectively, were prepared. Disk “A”, a reference disk, has a structure of 80nm ZnS-SiO2/15nm GeTeSb/ 20nm ZnS-SiO2 for layer 1. In addition to the structure of disk “A”, an additional optical switching structure of 30nm optical switching layer (silver oxide)/
20nm ZnS-SiO2 was fabricated as disk “B”. Transition temperature (Tt) of sample (Ar/O2:8/1) is about 90 oC as shown in Fig. 4.2.2 (a), temperature on the optical switching layer could be raised over transition temperature during recording (over 400
oC). The result reveals that optical switching layer could change recording characteristics of layer 1. The dependence of carrier-to-noise ratio (CNR) on the mark size of two sample disks was measured and shown in Fig. 4.3.1 by using a dynamic tester. The linear velocity was 7.4 m/s and the laser power was modulated by a single frequency pulse train from 2 to 8 MHz with 50% duty cycle. For sample disks “A”
and “B”, CNRs of 45 dB or higher were obtained at a peak power of 18.1 and 7.7 mW, respectively. The peak writing power in the disk “B” was reduced by 42% due to the additional optical switching layer. However, CNR dropped when the peak writing power increased, maybe due to that excessive laser energy overheated and slightly damaged the recording material. The result shows that, higher laser energy efficiency is obtained on a disk with an additional optical switching structure. The CNR as functions of direct overwrite (DOW) cycles with mark size of 0.6 µm of disks are shown in Fig. 4.3.2. The recording performance maintained on disk “B” before 2000 DOW cycles, which is applicable for DVD-RW applications.
DISK A
Fig. 4.3.1. CNR as functions of the mark size and peak write power of a disk (a) without and (b) with an optical switching layer.
38
Fig. 4.3.2. CNR as functions of number of DOW on disks “A” and “B”.
The above results show the peak writing power of a disk with additional optical switching structures can be reduced, consequently, the number of recording layers on a disk substrate can increase. To write on the lower recording layer, the output laser power needs to be properly increased to compensate for the loss in transmitting through the upper recording layers. To record lower recording layers, the output laser power was raised by a factor of T, the transmittance of the upper recording layers. The required writing power to record on the lower recording layer in both disks as a function of factor T is shown in Fig. 4.3.3. The measured transmittance in the as-deposited state of the disks “A” and “B” are 63.3% and 58.6%, respectively. Thus, average transmittance of the first recording layer of 60% was chosen in the following calculations. If the laser diode in the drive can provide 30 mW peak writing power, then 18 and 28 mW laser power is required for recording on the two recording layers, respectively. If a four-layer disk with optical switching structures in each layer, laser power of 7, 11, 17, 27 mW is calculated to record on the four recording layers,
respectively. Thus, multiple recording layers with optical switching structures can increase data capacity on a disk, as illustrated in Fig. 4.3.4.
0
Fig. 4.3.3. Simulated writing power to record on the second recording layer in both disks as a function of transmittance of the first recording layer.
11 mW
Fig. 4.3.4. Simulated writing power to record on each recording layer in disks (a) without and (b) with optical switching structures.
4.5 Summary
Optical switching layers in volumetric optical disks were proposed to modulate the
optical characteristics during writing. Materials, whose transmittance decreases with temperature, such as thermo-chromatic materials, are potential candidate as an optical switching layer. The silver oxide sample exhibiting transmittance change by 43% with increased temperature was chosen for the optical switching layer in the disk.
Experimental results showed the peak writing power could be reduced by 42% in the disk with an additional optical switching layer. Furthermore, Optical switching layer is promising for volumetric recording to optimize the writing laser power, thus R/W performance.
CHAPTER 5
WRITE-ONCE VOLUMETRIC OPTICLA DISK USING TRANSPARENT RECORDING MATERIAL WITH AN OPTICAL SWITCHING LAYER
A volumetric optical disk having multiple transparent films with optical switching layers is proposed as a recordable medium to increase the number of recording layers.
In the disk, an optical switching layer is adopted to reduce laser energy decay and increase recording sensitivity for reading and recording, respectively. A well-defined deformation area (mark) with submicron to nanometric dimensions can be yielded precisely on the transparent films by a focused laser beam. The peak writing power of 7 mW for a four-layer recordable medium, fabricated by molding and spin bonding techniques, was demonstrated experimentally as an example. The proposed volumetric disk can achieve a higher recording capacity by using conventional optical pickup units.
5.1 Introduction
As the demand for storage capacity continually grows, data storage technologies are being driven to achieve higher capacity, higher readout and recording bit rate, and faster access time. The recordable optical disk, as a two-dimensional optical storage medium, is currently the most widespread physical format for optical storage. A promising technique has been developed to increase the volumetric data density of optical disks by including a third physical dimension, using a multilayer recording
structure, i.e., by axially stacking a number of recording layers. However, issues such as interlayer crosstalk, low laser energy transmittance, and absorption exist in such a disk [5-1]. Therefore, the structure with an optical switching layer in a volumetric optical disk was introduced [5-2~3] to resolve these issues. In this chapter, a disk structure of multiple transparent films with an optical switching layer as recordable media is demonstrated to increase the number of recording layers in a volumetric optical disk.