Jochen Hellmig

Numerical simulation of mark formation in dual-stack phase-change recording

Dual-stack phase-change recording is an option to further increase the data capacity of rewritable optical disks. Such disks consist of two recording stacks that are both recorded and read from the same side of the disk. Consequently, the first recording stack needs therefore to be semitransparent to allow recording in the second recording stack. Thick nontransparent metal layers can therefore not be used in the first recording stack, which makes the first recording stack the most challenging stack from a thermal point of view. A numerical model based on crystal growth was developed to study formation and erasure of amorphous marks in phase-change stacks that are based on fast-growth doped SbTe phase-change materials. The validity of the model was demonstrated from transmission electron microscopy analyses of recorded marks that showed a good correspondence with the calculated mark shapes in a conventional single-stack recording stack. The model was subsequently applied to analyze formation and erasure of marks in slow-cooling phase-change stacks for digital versatile disk, (DVD) and digital video recording (DVR) recording conditions. The effect of the recording velocity, the erase power, and the crystal growth velocity on the erasability of amorphous marks was simulated. The calculated phenomena are in good agreement with the phenomena observed from DVD and DVR erasability measurements. Mark formation in slow-cooling recording stacks is characterized by severe recrystallization during writing. Two possible solutions are indicated in this article, aiming at reducing the heat accumulation and the resulting recrystallization during writing of amorphous marks. Additional semitransparent heat sinks improve the mark formation considerably but also require higher write powers. Another solution is the application of modified write strategies. Modeling and recorder results are discussed for both approaches.

Dual-Layer Blu-ray Disc Based on Fast-Growth Phase-Change Materials

A dual-layer disc is developed for blue-laser phase-change recording. The two recording stacks are based on fast-growth phase-change materials (FGMs). Both layers show good recording performance at a total disc capacity of 46.6 GB and with a slightly high jitter at 50 GB. The thicknesses of both the cover and spacer layers are controlled such that the deviations from the reference thicknesses of both recording layers are less than 2 µm. This eliminates the need for dynamical spherical aberration correction in the drive. The absolute difference in transmission between the written and unwritten states of the upper layer is only 2%. It is shown that this transmission difference causes no problem for readout and writing of the lower layer.

Phase-Change Media for Ultrahigh-Speed Digital Versatile Disc Recording

Rewritable discs based on Ge-Sn-Sb phase-change compositions that combine high crystallization rates with good archival life stability were designed. The crystallization rate can be varied from 2–16× digital versatile disc (DVD) speed by variation of phase-change composition. The rate increases with decreasing Ge concentration and is dependent on dopants. For high-speed discs based on these compositions, back growth is substantial, but can be diminished by designing an efficiently cooled stack. By doing so, phase-change optical recording at 10× DVD rewritable (+RW) with 9% multitrack jitter is demonstrated using a 2T-period-based write strategy.

Development of Recording Stacks for a Rewritable Dual-Layer Optical Disc

In this paper results are presented on the development of recording layers for a rewritable dual-layer phase-change optical disc. The disc is based on the digital video recording (DVR) system utilising a blue laser and a high numerical-aperture objective lens. With such a disc the capacity of the DVR system could be increased to about 45 GB. Various stack designs, write strategies and recording characteristics of the media are discussed. For each recording layer a design with a fairly good performance is found.

System Aspects of Dual-Layer Phase-Change Recording with High Numerical Aperture Optics and Blue Laser.

In this publication, key technologies for blue laser recording and dual layer phase-change disks with a total capacity of 50 Gbytes are explained and system margins are evaluated. Key challenges are the phase-change recording layer design, a robust read and write system with low intrinsic noise and the optical pickup design including, e.g., high numerical aperture (NA) objective lens, spherical aberration compensation devices and beam shaping optics. Blue-laser dual-layer technology will be emerging as the next-generation optical data storage standard beyond compact discs (CDs) and digital versatile discs (DVDs).

VCSEL based sensors for distance and velocity

VCSEL based sensors can measure distance and velocity in three dimensional space and are already produced in high quantities for professional and consumer applications. Several physical principles are used: VCSELs are applied as infrared illumination for surveillance cameras. High power arrays combined with imaging optics provide a uniform illumination of scenes up to a distance of several hundred meters. Time-of-flight methods use a pulsed VCSEL as light source, either with strong single pulses at low duty cycle or with pulse trains. Because of the sensitivity to background light and the strong decrease of the signal with distance several Watts of laser power are needed at a distance of up to 100m. VCSEL arrays enable power scaling and can provide very short pulses at higher power density. Applications range from extended functions in a smartphone over industrial sensors up to automotive LIDAR for driver assistance and autonomous driving. Self-mixing interference works with coherent laser photons scattered back into the cavity. It is therefore insensitive to environmental light. The method is used to measure target velocity and distance with very high accuracy at distances up to one meter. Single-mode VCSELs with integrated photodiode and grating stabilized polarization enable very compact and cost effective products. Besides the well know application as computer input device new applications with even higher accuracy or for speed over ground measurement in automobiles and up to 250km/h are investigated. All measurement methods exploit the known VCSEL properties like robustness, stability over temperature and the potential for packages with integrated optics and electronics. This makes VCSEL sensors ideally suited for new mass applications in consumer and automotive markets.

DVDs: in the fast lane

IMAGINE trying to burn submicron marks with nanometre precision onto a disk that is spinning at nearly 200 kilometres per hour. This feat can be likened to the challenges of building a Formula 1 racing car – the use of the latest technology in pursuit of speed. And like Formula 1 teams, the manufacturers of high-speed DVD drives have entered a fierce race to make sure that it is their name that is on the worlds fastest DVD recorder.

Advances in thermal modeling of dual-layer DVR-blue fast-growth media

Dual-layer phase-change recording is an option to further increase the data-capacity of re-writeable optical discs. A numerical model based on crystal growth was used to study formation and erasure of amorphous marks in dual-layer stacks based on fast-growth doped eutectic SbTe phase-change materials. The effect of the linear velocity, erase power, and crystal growth velocity on the erasability of amorphous marks was simulated. The calculated effects are in good agreement with the phenomena observed from DVD and DVR measurements. Mark formation in slow-cooling dual-layer stacks is characterized by severe re-crystallization during writing. Two possible solutions are indicated in the paper aimed at reducing the heat accumulation in the recording stack, and thus re-crystallization, during writing of amorphous marks. Additional transparent heat sinks improve the mark formation considerably but also require higher write powers. Another solution is based on an appropriate write strategy. Recording results obtained with this such a write strategy are shown.

High-Speed 16× Digital Versatile Disk Dye Recording

In dye-based optical recording, thermal interference is a major obstacle for high recording speeds. Detailed studies of recorded data reveal that post-heat from writing of a pit can result in unwanted shrinkage of the previously written pit. To prevent pit shrinkage we apply a castle-type writestrategy. We demonstrate digital versatile disk (DVD) recordings at 16× speed, 1× and intermediate speeds.

Rewritable dual-layer blu-ray disc media

The Blu-ray Disc (BD) optical recording system utilizes a blue laser (405 nm wavelength) in combination with a high-numerical-aperture (NA = 0.85) objective lens. This BD-format allows storing 23 - 27 GB on a single-layer and 46 - 54 GB on a dual-layer 12 cm disc at 36 Mbps user data rate. This paper reports on the design, manufacturing, and the recording performance of our dual-layer rewritable optical media. These media are based on the eutectic, fast-growth phase-change materials, which are promising candidates for high-speed applications. The performance of these materials in the case of dual-layer recording is discussed, with special attention for transmission balance for the semitransparent recording stack.