KEYWORDS: Modulation, Optical discs, Phase shift keying, Clocks, Objectives, Data conversion, Error analysis, Tolerancing, Signal to noise ratio, Signal detection
Recently, there has been much research on high-density recording of phase change optical disc using blue laser. In most research, RLL (Run Length Limited) modulation such as (1,7) RLL and EFM (Eight to Fourteen Modulation) modulation is used for high-density recording and channel clock regenerating. NRZI (Non Return to Zero Inverse) modulation has a wider window width than other modulation methods. This is a superior characteristic for achieving a high-density recording system of optical disc. We introduced NRZI+ modulation to suppress the low frequency components. And we obtained a result of a bER (bit Error Rate) of less than 1×10-4 at a bit length of 0.13μm using Turbo coding with an objective lens of NA 0.7 and a blue laser of 405 nm wavelength. This result shows that a recording capacity of 20 GB is feasible using NRZI modulation.
We have achieved a 2.5 inch (64 mm) quadruple density CD-ROM by developing a high resolution pick-up with our newly-developed visible laser diode whose wavelength is 635 nm, and also developing a disk cutting machine using a Kr ion laser with a wavelength of 407 nm.
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