Analysis of Multipulse Strategies for High Data Rate Phase Change Optical Recording
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Analysis of Multipulse Strategies for High Data Rate Phase Change Optical Recording Aparna C. Sheila, T.E. Schlesinger, DSSC, ECE Department, Carnegie Mellon University Pittsburgh, PA 15213, U.S.A ABSTRACT Multipulse strategies for two disk rotation velocities of the same phase change optical disk, were analyzed using simulations, at red and blue wavelengths. Results showed that at either wavelength, lower pulse duty cycles are needed at lower velocities to reduce recrystallization and higher duty cycles are needed at higher velocities. Optimal erase powers and cooling width for minimal overwrite jitter were also found. INTRODUCTION To achieve higher data rates in phase change (PC) optical recording, there are two options: one is to write smaller bits by decreasing the laser wavelength and/ or increasing the numerical aperture (NA) of the objective lens while the other option is to increase the disk rotation velocity for a given wavelength and NA. Using higher linear velocities is advantageous from the reliability and removability of the disk perspective, since the distance between the PC media and the lens has to be decreased when a high NA lens is used. In this paper, we look at some issues in achieving high data rates by increasing the disk rotation velocity. The maximum data rate achievable in a phase change recording material is limited by the time taken to crystallize or erase an amorphous mark. When constant angular velocity mode is used, different locations on the PC disk will be subjected to different linear velocities. The maximum velocities that give reasonable erasability depends on wavelength. We consider two velocities each, at red (640nm) and blue (430nm) wavelength and investigate how the multipulse parameters such as the pulse duty cycle, erase power and cooling width should be modified to give comparable performance at the two velocities. SIMULATION The time spent by any point on the disk under laser irradiation is referred to as the effective heating duration [1] th = d/v, where d is the laser spot diameter and v is the linear velocity. GeSbTe (GST) alloys crystallize through nucleation followed by grain growth. The minimum time for crystallization is therefore limited by nucleation time. It was shown that using the GST alloys, velocities upto 50m/s, with an effective heating duration of 17.5 ns would give reasonable erasability of 30 dB [1]. We assume that with crystallization enhancement layers on either side of the phase change layer, th can be 15ns. This gives for red (640nm) and 0.85 NA, a maximum velocity of 50m/s and for blue (430nm) with 0.85 NA, 33.5m/s. We numerically solved the thermal diffusion equation and the crystallization kinetics of nucleation and growth alternately in each time interval, taking into account the phase V1.6.1
changes that occur. The time interval between nucleation events was modeled as a random variable with an exponential density function. We assume two-dimensional nucleation and growth. The details of the simulation are given in Ref.2. The intensity of a read
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